WIP: Alternate data layout Salsa20 for (ye)scrypt

[eternal-flame-AD]

This should not affect soft core performance as Salsa20 is designed to be register resident and this is just register wiring.

TODO: pull in an SSE2 and AVX2 dual-buffer core and statically dispatch in place of the soft core when round count is high enough to be profitable: https://github.com/eternal-flame-AD/scrypt-opt/blob/main/src/salsa20/x86_64.rs

Before:

test salsa12_bench1_16b   ... bench:          12.43 ns/iter (+/- 0.61) = 1333 MB/s
test salsa12_bench2_256b  ... bench:         159.03 ns/iter (+/- 12.04) = 1610 MB/s
test salsa12_bench3_1kib  ... bench:         619.33 ns/iter (+/- 6.29) = 1654 MB/s
test salsa12_bench4_16kib ... bench:       9,915.67 ns/iter (+/- 38.58) = 1652 MB/s
test salsa20_bench1_16b   ... bench:          18.40 ns/iter (+/- 0.23) = 888 MB/s
test salsa20_bench2_256b  ... bench:         253.31 ns/iter (+/- 11.76) = 1011 MB/s
test salsa20_bench3_1kib  ... bench:       1,001.52 ns/iter (+/- 7.56) = 1022 MB/s
test salsa20_bench4_16kib ... bench:      15,858.24 ns/iter (+/- 83.23) = 1033 MB/s
test salsa8_bench1_16b    ... bench:           9.59 ns/iter (+/- 0.13) = 1777 MB/s
test salsa8_bench2_256b   ... bench:         112.39 ns/iter (+/- 2.31) = 2285 MB/s
test salsa8_bench3_1kib   ... bench:         433.26 ns/iter (+/- 13.06) = 2364 MB/s
test salsa8_bench4_16kib  ... bench:       6,801.70 ns/iter (+/- 53.38) = 2409 MB/s

After:

test salsa12_bench1_16b   ... bench:          12.68 ns/iter (+/- 0.44) = 1333 MB/s
test salsa12_bench2_256b  ... bench:         158.09 ns/iter (+/- 5.91) = 1620 MB/s
test salsa12_bench3_1kib  ... bench:         617.26 ns/iter (+/- 15.70) = 1659 MB/s
test salsa12_bench4_16kib ... bench:       9,713.60 ns/iter (+/- 247.77) = 1686 MB/s
test salsa20_bench1_16b   ... bench:          18.43 ns/iter (+/- 1.11) = 888 MB/s
test salsa20_bench2_256b  ... bench:         255.13 ns/iter (+/- 11.43) = 1003 MB/s
test salsa20_bench3_1kib  ... bench:         994.71 ns/iter (+/- 4.67) = 1030 MB/s
test salsa20_bench4_16kib ... bench:      15,787.80 ns/iter (+/- 47.00) = 1037 MB/s
test salsa8_bench1_16b    ... bench:           9.74 ns/iter (+/- 0.13) = 1777 MB/s
test salsa8_bench2_256b   ... bench:         113.25 ns/iter (+/- 3.86) = 2265 MB/s
test salsa8_bench3_1kib   ... bench:         437.69 ns/iter (+/- 14.69) = 2343 MB/s
test salsa8_bench4_16kib  ... bench:       6,752.95 ns/iter (+/- 101.23) = 2426 MB/s

test salsa8_bench1_ks      ... bench:          34.39 ns/iter (+/- 0.57) = 1882 MB/s
test salsa8_bench1_ks_altn ... bench:          29.47 ns/iter (+/- 3.24) = 2206 MB/s

xref: RustCrypto/password-hashes#622