On your suggestion, I tried this out. However, it seemed to hurt the performance of Sha512:
```
test sha2::bench::sha256_10 ... bench: 93 ns/iter (+/- 4) = 107 MB/s
test sha2::bench::sha256_1k ... bench: 7054 ns/iter (+/- 433) = 145 MB/s
test sha2::bench::sha256_64k ... bench: 447231 ns/iter (+/- 5512) = 146 MB/s
test sha2::bench::sha512_10 ... bench: 65 ns/iter (+/- 1) = 153 MB/s
test sha2::bench::sha512_1k ... bench: 4415 ns/iter (+/- 79) = 231 MB/s
test sha2::bench::sha512_64k ... bench: 280107 ns/iter (+/- 3000) = 233 MB/s
```