In this commit, we add a new signature type. We’ll use this type to
avoid fully decoding a signature on the wire into a btcec.Signature.
This type is only really needed when we need to do signature
validation, as a result, always encoding it is a waste. Several helper
methods have been added to the new struct in order to ensure that we
can use it in the existing codebase without substantial issues.
This commit modifies the encoding of signatures on the wire to use
a fixed-size 64-byte format. This change is required as the current spec
draft dictates that all signatures be encoded as `R` and `S` as 32-byte
big-endian integers. With this, signatures are now always a _fixed_ size
slice of bytes on the wire, which is nice to have.
Fixes#83.
