keccak: avoid materializing round constants array

[shuklaayush]

• Switch from eager array .map() to lazy iterator mapping for round constants
• Avoid materializing a temporary [L; ROUNDS], reducing stack usage

[newpavlov]

With enabled optimization this code gets optimized into a pre-computed const table, so this change is not necessary.

[shuklaayush]

@newpavlov thanks for the reply! the conversions are constant-folded into a table, but the owned array is still materialized in the optimized aarch64 codegen

before:

subsp,sp, #0x190ldp q0, q1,[x8, #0x80]stp q0, q1,[sp, #0xf0]ldp q0, q1,[x8, #0xa0]stp q0, q1,[sp, #0x110]ldp q0, q1,[x8, #0x40]stp q0, q1,[sp, #0xb0]ldp q0, q1,[x8, #0x60]stp q0, q1,[sp, #0xd0]ldp q0, q1,[x8]stp q0, q1,[sp, #0x70]ldp q0, q1,[x8, #0x20]stp q0, q1,[sp, #0x90]

after:

subsp,sp, #0xc0adrp x9, ...add x9, x9, ...ldr x9,[x9, x8]

on my macbook air m3, running

RUSTFLAGS='--cfg keccak_backend="soft"' \
cargo +nightly bench -p keccak --bench mod -- keccak_f1600 --exact

goes from 172.63 ns before to 145.93 ns after

[newpavlov]

Interesting. I opened #129 to resolve it in a different and hopefully less fragile fashion.