Compatibility with RustCrypto

[daxpedda]

Hi!

This PR makes a couple of changes with the goal to make this library compatible with an upcoming Rust implementation in RustCrypto: RustCrypto/password-hashes#232

There are really only two changes required that intend to bring the implementation closer to the research paper/prototype implementation:

1. Values are currently all converted to UTF-8 strings before being hashed, the change improves on that by converting all integers to little-endian 64-bit byte representation and values that are already bytes are passed through instead of converted to strings.
2. The research paper mentions a ints_to_block function, but otherwise doesn't go into detail what this is. The prototype implementation uses encryption to create a byte "block" of these values. I have decided to simply hash them with the selected hash. Alternatively it may be valuable to always use SHA-1 here as this part isn't really cryptographically important and it could avoid any increased cost by "slower" hashes.

I'm open to any suggestions!

[nachonavarro]

Hey @daxpedda this is great! Thank you for taking the time, I'll merge now.

[daxpedda]

Amazing, thank you!