Rewrite the base64 decoder

Rewrite the base64 decoder

The old decoding was done using libcrypto's BIO API, even though the
code didn't need to perform any stream chaining. (Which seems to be the
whole point.)

Trying to figure out why it wasn't properly erroring on invalid base64,
I eventually realized I could dramatically simplify everything by
shifting the API layer. I'm now using `EVP_DecodeUpdate()` and friends.
It's so much cleaner and more correct now; it's incredible.

This had a cascade effect. The complexity of the base64.h API also
collapsed, which resulted in massive refactors on the callers.

Also adds some unit tests for base64 and TAL.