Support decoding RFC 6979 signatures to (r, s)

This PR will add `pyasn1` as a hard dependency (and removes it from test deps). It adds the ability to encode `(r, s)` to RFC 6979 signatures as well as decoding back to `(r, s)`

Fixes #1285.
So, thinking a bit more, this is really only intended to be used with the results of our own signers -- which is to say, there should never be stray bytes, so I think we should do the check.