auto merge of #8272 : DaGenix/rust/digest-md5-impl-not-unrolled, r=cmr

An MD5 implementation was originally included in #8097, but, since there are a couple different implementations of that digest algorithm (@alco mentioned his implementation on the mailing list just before I opened that PR), it was suggested that I remove it from that PR and open up a new PR to discuss the different implementations and the best way forward. If anyone wants to discuss a different implementation, feel free to present it here and discuss and compare it to this one. I'll just discuss my implementation and I'll leave it to others to present details of theirs.

This implementation relies on the FixedBuffer struct from cryptoutil.rs for managing the input buffer, just like the Sha1 and Sha2 digest implementations do. I tried manually unrolling the loops in the compression function, but I got slightly worse performance when I did that.

Outside of the #[test]s, I also tested the implementation by generating 1,000 inputs of up to 10MB in size and checking the MD5 digest calculated by this code against the MD5 digest calculated by Java's implementation.

On my computer, I'm getting the following performance:

```
test md5::bench::md5_10 ... bench: 52 ns/iter (+/- 1) = 192 MB/s
test md5::bench::md5_1k ... bench: 2819 ns/iter (+/- 44) = 363 MB/s
test md5::bench::md5_64k ... bench: 178566 ns/iter (+/- 4927) = 367 MB/s
```

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