Add support for unencoded hash

[laerling]

Hello.

Sometimes the raw bytes of a hash are needed instead of the hexadecimal representation. For example for double-hashing.
Currently to achieve this I need to use the hex crate and call hex::decode(sha256::digest(...)).expect(...), which is an unnecessary layer of indirection.
The usual way to implement this is to have two separate methods, for example digest (yielding raw bytes) and hexdigest (yielding a string containing the hexadecimal representation).

Thanks to the Cargo.lock mechanism, backwards compatibility is not a technical issue and so the existing digest function could be renamed to hexdigest and another digest function added that yields &[u8] or [u8; 32].
But if you prefer the digest function to stay untouched for backwards compatibility, I suggest adding a function called for example digest_raw or digest_unencoded.

I'm willing to try making a PR but wanted to ask for your thoughts on this request beforehand.
Thank you for your effort!

[baoyachi]

@laerling Very good suggestion, looking forward to your PR.