Add x86 intrinsics support for sha2-512

[Rexagon]

This version is a port of this implementation (src/sha512_compress_x86_64_avx2.c) https://github.com/aws-samples/sha2-with-c-intrinsic which is based on this document https://eprint.iacr.org/2012/067.pdf. It uses avx impl for single odd block and then avx2 for the rest.

Benchmarks:

CPU: Ryzen 9 5950X

cargo +nightly bench -p sha2 --features force-soft

     Running unittests (target/release/deps/sha512-0451eb22388b5224)

running 4 tests
test bench1_10    ... bench:          22 ns/iter (+/- 0) = 454 MB/s
test bench2_100   ... bench:         184 ns/iter (+/- 8) = 543 MB/s
test bench3_1000  ... bench:       1,869 ns/iter (+/- 5) = 535 MB/s
test bench4_10000 ... bench:      18,226 ns/iter (+/- 54) = 548 MB/s

test result: ok. 0 passed; 0 failed; 0 ignored; 4 measured; 0 filtered out; finished in 5.02s

cargo +nightly bench -p sha2

     Running unittests (target/release/deps/sha512-fc38283933ec7290)

running 4 tests
test bench1_10    ... bench:          18 ns/iter (+/- 0) = 555 MB/s
test bench2_100   ... bench:         148 ns/iter (+/- 1) = 675 MB/s
test bench3_1000  ... bench:       1,317 ns/iter (+/- 11) = 759 MB/s
test bench4_10000 ... bench:      12,836 ns/iter (+/- 156) = 779 MB/s

test result: ok. 0 passed; 0 failed; 0 ignored; 4 measured; 0 filtered out; finished in 3.06s

[Rexagon]

Hopefully related to #311

[tarcieri]

Looks good to me.

@newpavlov WDYT?

[newpavlov]

I haven't looked deep into implementation itself and would like to try some minor tweaks, but I think we can merge it after the target_feature(enable = "..") nitpick will be fixed.

[newpavlov]

Thank you! I will soon release it as v0.9.7