In this commit, we optimize our signature implementation slightly, by
defining pre-computed sha256(tag) variables for the commonly used
values. If a tag matches this, then we'll use that hash value to avoid
an extra round of hashing.
In this commit, we add an implementation of the BIP-340 tagged hash
scheme. This initial version can be optimized quite a bit, for example,
we can hard code the output of frequently used `sha256(tag)` values and
save two `sha256` invocations.
This is a backport of the chainhash package made in Decred along with a
few additional things cleaned up, finished test coverage, and rewording
of some documentation to make it more generic.
In particular, the new package provides the definition of the hash type
and associated hashing functions which will allow the rest of the code to be
agnostic to the specific hash algorithm.
This only implements the package and does not change any of the code
base over to use it.
