Using RustCrypto `Digest` trait directly

[vmx]

Most hashers implement RustCrypto's Digest trait. Once RustCrypto/hashes#228 lands all hashers except for strobe implement the Digest trait. We could then think about using the Digest trait directly instead of having our own custom hashers defined. Probably we would still export some type aliases for convenience/backwards compatibility.

[thomaseizinger]

Cross-referencing a discussion: #272 (comment)