Rfc6979

Implement RFC 6979 deterministic signatures. This also indroduces the data_slice abstraction, which makes data_chunk and byte_array interchangeable for use as hash parameters.
I was the one who added that line, simultaneously with submitting a pull request to upstream. We also have a unit test that covers this case. So, now that upstream has moved forward (definitely so, because we require their new API), we can safely remove this line.