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 initial implementation of BIP-340. Mirroring
the recently added `ecsda` package, we create a new `schnorr` package
with a unique `Signature` type and `ParsePubkey` function. The new
`Signature` type implements the fixed-sized 64-byte signatures, and the
`ParsePubkey` method only accepts pubkeys that are 32-bytes in length,
with an implicit sign byte.
The signing implementation by default, deviates from BIP-340 as it opts
to use rfc6979 deterministic signatures by default, which means callers
don't need to always pass in their own `auxNonce` randomness. A set of
functional arguments allows callers to pass in their own value, which is
the way all the included test vectors function.
The other optional functional argument added is the `FastSign` option
that allows callers to skip the final step of verifying each signature
they generate.
