This also simplifies the verification logic. Because the verification check
happens in variable time, we don't need to do a constant-time eq check at the
end, so we can drop the `subtle` dependency entirely.
The `DecodingError` type becomes `SignatureError` and is also used to
signal failing verifications.
This code was significantly based off without boats' error types in
commit 6c1acaca7c, and also upon
conversation with them. Please target them with praise, and blame me
for whatever mistakes I might have made.
