Make unsound opt-in feature

[JackThomson2]

Description

Updating of the PR: #51
Rebased against current main and re-ran the performance tests to compare the difference.

Unsure if the unsound should be opt-in or opt-out, but left match previous PR for now

Tested on c7g (Graviton 3)

 name safe ns/iter unsound ns/iter diff ns/iter diff % speedup benches::bench_bincode_deserialize 182,927 178,688 -4,239 -2.32% x 1.02 benches::bench_bincode_serialize 24,170 24,311 141 0.58% x 0.99 benches::bench_bitcode_decode 131,508 131,705 197 0.15% x 1.00 benches::bench_bitcode_encode 16,323 14,305 -2,018 -12.36% x 1.14 benches_borrowed::bench_bincode_deserialize 91,928 91,571 -357 -0.39% x 1.00 benches_borrowed::bench_bincode_serialize 23,047 23,035 -12 -0.05% x 1.00 benches_borrowed::bench_bitcode_decode 12,158 12,179 21 0.17% x 1.00 benches_borrowed::bench_bitcode_encode 16,477 14,599 -1,878 -11.40% x 1.13 bool::test2::bench_bool_vecs_decode 4,988 4,968 -20 -0.40% x 1.00 bool::test2::bench_bool_vecs_encode 365 226 -139 -38.08% x 1.62 bool::test::bench_bool_vec_decode 92 91 -1 -1.09% x 1.01 bool::test::bench_bool_vec_encode 61 61 0 0.00% x 1.00 buffer::tests::bench_registry100_get 19 19 0 0.00% x 1.00 buffer::tests::bench_registry10_get 14 14 0 0.00% x 1.00 buffer::tests::bench_registry1_get 1 1 0 0.00% x 1.00 derive::array::tests::bench_u8_array_vecs_decode 4,480 4,512 32 0.71% x 0.99 derive::array::tests::bench_u8_array_vecs_encode 431 436 5 1.16% x 0.99 derive::duration::tests::bench_duration_vec_decode 833 837 4 0.48% x 1.00 derive::duration::tests::bench_duration_vec_encode 1,094 1,099 5 0.46% x 1.00 derive::empty::tests::bench_phantom_data_vec_decode 8 8 0 0.00% x 1.00 derive::empty::tests::bench_phantom_data_vec_encode 12 12 0 0.00% x 1.00 derive::impls::tests::bench_tuple_vec_decode 10,250 10,248 -2 -0.02% x 1.00 derive::impls::tests::bench_tuple_vec_encode 8,975 8,976 1 0.01% x 1.00 derive::map::test::bench_btree_map_decode 2,009 2,012 3 0.15% x 1.00 derive::map::test::bench_btree_map_encode 661 662 1 0.15% x 1.00 derive::map::test::bench_hash_map_decode 3,434 3,486 52 1.51% x 0.99 derive::map::test::bench_hash_map_encode 655 660 5 0.76% x 0.99 derive::option::tests2::bench_option_u16_vec_decode 769 762 -7 -0.91% x 1.01 derive::option::tests2::bench_option_u16_vec_encode 703 703 0 0.00% x 1.00 derive::option::tests::bench_option_vec_decode 7,257 7,261 4 0.06% x 1.00 derive::option::tests::bench_option_vec_encode 6,349 6,344 -5 -0.08% x 1.00 derive::result::tests::bench_result_vec_decode 907 912 5 0.55% x 0.99 derive::result::tests::bench_result_vec_encode 2,193 2,229 36 1.64% x 0.98 derive::variant::tests::bench_bool_enum_vec_decode 105 102 -3 -2.86% x 1.03 derive::variant::tests::bench_bool_enum_vec_encode 89 89 0 0.00% x 1.00 derive::vec::test::bench_binary_heap_decode 208 209 1 0.48% x 1.00 derive::vec::test::bench_btree_set_decode 1,436 1,435 -1 -0.07% x 1.00 derive::vec::test::bench_btree_set_encode 368 370 2 0.54% x 0.99 derive::vec::test::bench_hash_set_decode 3,234 3,236 2 0.06% x 1.00 derive::vec::test::bench_hash_set_encode 310 310 0 0.00% x 1.00 derive::vec::test::bench_linked_list_decode 5,481 5,467 -14 -0.26% x 1.00 derive::vec::test::bench_linked_list_encode 458 458 0 0.00% x 1.00 derive::vec::test::bench_vec_decode 41 41 0 0.00% x 1.00 derive::vec::test::bench_vec_deque_decode 41 41 0 0.00% x 1.00 derive::vec::test::bench_vec_deque_encode 43 43 0 0.00% x 1.00 derive::vec::test::bench_vec_encode 41 41 0 0.00% x 1.00 f32::tests2::bench_f32_vecs_decode 4,582 4,595 13 0.28% x 1.00 f32::tests2::bench_f32_vecs_encode 650 681 31 4.77% x 0.95 f32::tests::bench_f32_vec_decode 454,806 456,517 1,711 0.38% x 1.00 f32::tests::bench_f32_vec_encode 715,469 715,775 306 0.04% x 1.00 fast::tests::bench_next_unchecked 3,465 3,465 0 0.00% x 1.00 fast::tests::bench_next_unchecked_fast 1,930 1,931 1 0.05% x 1.00 fast::tests::bench_push 2,445 2,455 10 0.41% x 1.00 fast::tests::bench_push_fast 4,285 4,288 3 0.07% x 1.00 fast::tests::bench_push_unchecked 2,813 2,864 51 1.81% x 0.98 fast::tests::bench_push_unchecked_fast 3,728 3,721 -7 -0.19% x 1.00 fast::tests::bench_reserve 691 692 1 0.14% x 1.00 fast::tests::bench_reserve_fast 585 584 -1 -0.17% x 1.00 histogram::tests::bench_histogram_parallel_10 185 185 0 0.00% x 1.00 histogram::tests::bench_histogram_parallel_100 247 246 -1 -0.40% x 1.00 histogram::tests::bench_histogram_parallel_1000 847 846 -1 -0.12% x 1.00 histogram::tests::bench_histogram_parallel_10000 6,864 6,865 1 0.01% x 1.00 histogram::tests::bench_histogram_simple_10 54 54 0 0.00% x 1.00 histogram::tests::bench_histogram_simple_100 162 162 0 0.00% x 1.00 histogram::tests::bench_histogram_simple_1000 1,193 1,191 -2 -0.17% x 1.00 histogram::tests::bench_histogram_simple_10000 11,403 11,402 -1 -0.01% x 1.00 int::test2::bench_u16_vecs_decode 3,962 3,964 2 0.05% x 1.00 int::test2::bench_u16_vecs_encode 290 201 -89 -30.69% x 1.44 int::tests::bench_u16_vec_decode 111 109 -2 -1.80% x 1.02 int::tests::bench_u16_vec_encode 94 95 1 1.06% x 0.99 pack::tests::bench_pack_arithmetic16 48 48 0 0.00% x 1.00 pack::tests::bench_pack_arithmetic2 26 26 0 0.00% x 1.00 pack::tests::bench_pack_arithmetic3 102 102 0 0.00% x 1.00 pack::tests::bench_pack_arithmetic4 44 44 0 0.00% x 1.00 pack::tests::bench_pack_arithmetic6 52 52 0 0.00% x 1.00 pack::tests::bench_unpack_arithmetic16 25 25 0 0.00% x 1.00 pack::tests::bench_unpack_arithmetic2 38 38 0 0.00% x 1.00 pack::tests::bench_unpack_arithmetic3 148 148 0 0.00% x 1.00 pack::tests::bench_unpack_arithmetic4 41 41 0 0.00% x 1.00 pack::tests::bench_unpack_arithmetic6 90 90 0 0.00% x 1.00 pack::tests::bench_unpack_bytes_less_than16 7 7 0 0.00% x 1.00 pack::tests::bench_unpack_bytes_less_than16_histogram 8 8 0 0.00% x 1.00 pack::tests::bench_unpack_bytes_less_than2 7 7 0 0.00% x 1.00 pack::tests::bench_unpack_bytes_less_than256 3 3 0 0.00% x 1.00 pack::tests::bench_unpack_bytes_less_than256_histogram 19 19 0 0.00% x 1.00 pack::tests::bench_unpack_bytes_less_than2_histogram 8 8 0 0.00% x 1.00 pack::tests::bench_unpack_bytes_less_than3 7 7 0 0.00% x 1.00 pack::tests::bench_unpack_bytes_less_than3_histogram 12 12 0 0.00% x 1.00 pack::tests::bench_unpack_bytes_less_than4 7 7 0 0.00% x 1.00 pack::tests::bench_unpack_bytes_less_than4_histogram 8 8 0 0.00% x 1.00 pack::tests::bench_unpack_bytes_less_than6 7 7 0 0.00% x 1.00 pack::tests::bench_unpack_bytes_less_than6_histogram 11 11 0 0.00% x 1.00 pack::tests::bench_unpack_histogram16 461 460 -1 -0.22% x 1.00 pack::tests::bench_unpack_histogram3 791 782 -9 -1.14% x 1.01 pack::tests::bench_unpack_histogram4 680 677 -3 -0.44% x 1.00 pack::tests::bench_unpack_histogram6 571 569 -2 -0.35% x 1.00 pack_ints::tests::bench_pack_u008_max 100 100 0 0.00% x 1.00 pack_ints::tests::bench_pack_u008_no_pack 35 35 0 0.00% x 1.00 pack_ints::tests::bench_pack_u008_random 74 74 0 0.00% x 1.00 pack_ints::tests::bench_pack_u008_zero 69 69 0 0.00% x 1.00 pack_ints::tests::bench_pack_u016_max 182 183 1 0.55% x 0.99 pack_ints::tests::bench_pack_u016_no_pack 59 59 0 0.00% x 1.00 pack_ints::tests::bench_pack_u016_random 67 68 1 1.49% x 0.99 pack_ints::tests::bench_pack_u016_zero 154 154 0 0.00% x 1.00 pack_ints::tests::bench_pack_u032_max 290 290 0 0.00% x 1.00 pack_ints::tests::bench_pack_u032_no_pack 112 112 0 0.00% x 1.00 pack_ints::tests::bench_pack_u032_random 121 122 1 0.83% x 0.99 pack_ints::tests::bench_pack_u032_zero 215 215 0 0.00% x 1.00 pack_ints::tests::bench_pack_u064_max 506 510 4 0.79% x 0.99 pack_ints::tests::bench_pack_u064_no_pack 216 211 -5 -2.31% x 1.02 pack_ints::tests::bench_pack_u064_random 224 220 -4 -1.79% x 1.02 pack_ints::tests::bench_pack_u064_zero 353 359 6 1.70% x 0.98 pack_ints::tests::bench_pack_u128_max 2,155 2,153 -2 -0.09% x 1.00 pack_ints::tests::bench_pack_u128_no_pack 477 450 -27 -5.66% x 1.06 pack_ints::tests::bench_pack_u128_random 476 474 -2 -0.42% x 1.00 pack_ints::tests::bench_pack_u128_zero 1,747 1,744 -3 -0.17% x 1.00 pack_ints::tests::bench_pack_usize_max 506 511 5 0.99% x 0.99 pack_ints::tests::bench_pack_usize_no_pack 211 211 0 0.00% x 1.00 pack_ints::tests::bench_pack_usize_random 225 220 -5 -2.22% x 1.02 pack_ints::tests::bench_pack_usize_zero 353 359 6 1.70% x 0.98 pack_ints::tests::bench_unpack_u008_max 62 62 0 0.00% x 1.00 pack_ints::tests::bench_unpack_u008_random 3 3 0 0.00% x 1.00 pack_ints::tests::bench_unpack_u008_zero 44 43 -1 -2.27% x 1.02 pack_ints::tests::bench_unpack_u016_max 111 111 0 0.00% x 1.00 pack_ints::tests::bench_unpack_u016_random 4 4 0 0.00% x 1.00 pack_ints::tests::bench_unpack_u016_zero 81 81 0 0.00% x 1.00 pack_ints::tests::bench_unpack_u032_max 190 189 -1 -0.53% x 1.01 pack_ints::tests::bench_unpack_u032_random 4 4 0 0.00% x 1.00 pack_ints::tests::bench_unpack_u032_zero 106 105 -1 -0.94% x 1.01 pack_ints::tests::bench_unpack_u064_max 313 313 0 0.00% x 1.00 pack_ints::tests::bench_unpack_u064_random 4 4 0 0.00% x 1.00 pack_ints::tests::bench_unpack_u064_zero 152 152 0 0.00% x 1.00 pack_ints::tests::bench_unpack_u128_max 1,185 1,184 -1 -0.08% x 1.00 pack_ints::tests::bench_unpack_u128_random 4 4 0 0.00% x 1.00 pack_ints::tests::bench_unpack_u128_zero 769 769 0 0.00% x 1.00 pack_ints::tests::bench_unpack_usize_max 313 313 0 0.00% x 1.00 pack_ints::tests::bench_unpack_usize_random 4 4 0 0.00% x 1.00 pack_ints::tests::bench_unpack_usize_zero 152 152 0 0.00% x 1.00 str::tests2::bench_str_vec_decode 1,374,455 1,369,288 -5,167 -0.38% x 1.00 str::tests2::bench_str_vec_encode 96,575 57,505 -39,070 -40.46% x 1.68 str::tests::bench_is_ascii 405 405 0 0.00% x 1.00 str::tests::bench_is_ascii_simd 421 416 -5 -1.19% x 1.01 str::tests::bench_str_tuple_decode 59 59 0 0.00% x 1.00 str::tests::bench_str_tuple_encode 92 92 0 0.00% x 1.00