In this commit, we implement the new BIP 341+342 taproot sighash digest
computation. The digest is similar, but re-orders some fragments and
also starts to commit to the input values of all the transactions in the
SIGHASH_ALL case. A new implicit sighash flag, SIGHASH_DEFAULT has been
added that allows signatures to always be 64-bytes for the common case.
The hashcache has been updated as well to store both the v0 and v1 mid
state hashes. The v0 hashes are a double-sha of the contents, while the
v1 hash is a single sha. As a result, if a transaction spends both v0
and v1 inputs, then we 're able to re-use all the intermediate hashes.
As the sighash computation needs the input values and scripts, we create
an abstraction: the PrevOutFetcher to give the caller flexibility w.r.t
how this is done. We also create a `CannedPrevOutputFetcher` that holds
the information in a map for a single input.
A series of function options are also added to allow re-use of the same
base sig hash calculation for both BIP 341 and 342.
This commit implements most of BIP0143 by adding logic to implement the
new sighash calculation, signing, and additionally introduces the
HashCache optimization which eliminates the O(N^2) computational
complexity for the SIGHASH_ALL sighash type.
The HashCache struct is the equivalent to the existing SigCache struct,
but for caching the reusable midstate for transactions which are
spending segwitty outputs.
