Comparison to standard libary's std::hash::Hash derive

[daboross]

Hi!

I came across this library from https://www.dfns.co/article/unambiguous-hashing.

Loved the writeup, but I felt like it was missing a comparison to existing alternatives - mainly, std::hash::Hash, and #[derive(Hash)]. I thought I'd suggest adding it!

Mainly, I was at first worried that maybe std::hash::Hash is vulnerable to moving around bytes like the naive implementation, found that it wasn't, but that (surprisingly to me) it is platform-dependent - impl Hash for [u8] encodes length as a usize, and thus so does Vec and String. I think that's a great selling point for udigest.