BTree: merge the complication introduced by #81486 and #86031

[ssomers]

Also:

• Deallocate the last few tree nodes as soon as an into_iter iterator steps beyond the end, instead of waiting around for the drop of the iterator (just to share more code).
• Symmetric code for backward iteration.
• Mark unsafe the methods on dying handles, modelling dying handles after raw pointers: it's the caller's responsibility to use them safely.

r? @Mark-Simulacrum

[ssomers]

Alloc benchmarks show meaningless shifting around:

benchcmp old new --threshold 10
name old ns/iter new ns/iter diff ns/iter diff % speedup
btree::map::clone_fat_val_100_and_into_iter 10,882 14,365 3,483 32.01% x 0.76
btree::map::clone_slim_100 4,424 1,470 -2,954 -66.77% x 3.01
btree::map::clone_slim_10k_and_into_iter 185,360 211,358 25,998 14.03% x 0.88
btree::map::find_seq_100 14 11 -3 -21.43% x 1.27
btree::map::first_and_last_10k_nightly 61 68 7 11.48% x 0.90
btree::map::iteration_20 53 60 7 13.21% x 0.88
btree::map::iteration_mut_1000 3,723 4,173 450 12.09% x 0.89
btree::map::iteration_mut_20 57 67 10 17.54% x 0.85
btree::set::difference_random_100_vs_10k 2,538 2,867 329 12.96% x 0.89
btree::set::difference_staggered_100_vs_10k 2,208 2,724 516 23.37% x 0.81
btree::set::intersection_random_100_vs_100 786 702 -84 -10.69% x 1.12
btree::set::intersection_random_100_vs_10k 2,290 2,576 286 12.49% x 0.89
btree::set::intersection_random_10k_vs_100 2,270 2,620 350 15.42% x 0.87
btree::set::intersection_staggered_100_vs_100 779 676 -103 -13.22% x 1.15
btree::set::intersection_staggered_100_vs_10k 1,984 2,484 500 25.20% x 0.80
btree::set::intersection_staggered_10k_vs_10k 75,290 65,717 -9,573 -12.71% x 1.15
btree::set::is_subset_100_vs_100 731 643 -88 -12.04% x 1.14
btree::set::is_subset_100_vs_10k 1,771 1,546 -225 -12.70% x 1.15
btree::set::is_subset_10k_vs_10k 72,535 64,088 -8,447 -11.65% x 1.13

[Mark-Simulacrum]

@bors r+ rollup=never

[bors]

📌 Commit 7b28036 has been approved by Mark-Simulacrum

[bors]

⌛ Testing commit 7b28036 with merge 23461b2...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing 23461b2 to master...