This commit was previously split into the following parts to ease
review:
- 2d746f68: replace imports
- 4008f0fd: use ecdsa.Signature
- 849e33d1: remove btcec.S256()
- b8f6ebbd: use v2 library correctly
- fa80bca9: bump go modules
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.
