Add a `base64` or `encoding` feature and make the `base64` crate optional

[mrijkeboer]

Hi @dullbananas,

What’s your use-case for making the base64 dependency optional?

[dullbananas]

I only need hash_raw and verify_raw, which don't use base 64.

I don't want to download and compile any crates that I don't need because it increases compile time and the chance of downloading a malicious crate.