digest: improve documentation and test examples in readme

[newpavlov]

Lack of readme example tests has caused #1072.

Also we probably should unify documentation from the hashes and digest' readmes. I think of having most of docs in the digest's readme, which will be included into digest's docs, and we will reference those examples from the hashes readme and hash implementation crate-level docs.

Including working hash tests into digest's docs will cause a weird dependency cycle digest <- sha2 <- digest, but I think it should work since sha2 and other crates will be dev-dependencies.

[tarcieri]

Here's one approach I've taken to doctesting READMEs by creating a tiny cargo project with the necessary dependencies that embeds the README.md: https://github.com/RustCrypto/password-hashes/tree/master/.readme