Add portable SIMD implementation of blake2b.

[brndnmtthws]

This resolves #6.

[codecov]

Codecov Report

Merging #20 (e5802a0) into main (681ed58) will decrease coverage by 7.15%.
The diff coverage is 13.53%.

❗ Current head e5802a0 differs from pull request most recent head 7265fc2. Consider uploading reports for the commit 7265fc2 to get more accurate results

@@            Coverage Diff             @@
##             main      #20      +/-   ##
==========================================
- Coverage   72.53%   65.38%   -7.16%     
==========================================
  Files          42       44       +2     
  Lines        2851     3163     +312     
==========================================
  Hits         2068     2068              
- Misses        783     1095     +312     

Impacted Files	Coverage Δ
src/blake2b/blake2b_simd.rs	0.00% <0.00%> (ø)
src/lib.rs	16.66% <ø> (-25.01%)	⬇️
src/poly1305/u130.rs	0.00% <0.00%> (ø)
src/utils.rs	96.15% <91.66%> (+11.53%)	⬆️
src/argon2.rs	93.81% <100.00%> (ø)
src/blake2b/blake2b_soft.rs	89.41% <100.00%> (ø)
src/classic/crypto_core.rs	99.23% <100.00%> (ø)
src/poly1305/poly1305_soft.rs	98.55% <100.00%> (ø)
src/siphash24.rs	100.00% <100.00%> (ø)
... and 1 more

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