In this commit, we update all the taproot scripts to also accept an
optional aux leaf. This aux leaf can be used to add more redemption
paths for advanced channels, or just as an extra commitment space.
Because the original dcrec secp256k1 library that is used for the
Schnorr signature primitives uses different hash algorithms than the
btcd secp256k1 library. Therefore pulling in the wrong library can lead
to weird and unexpected errors. We try to make it harder to make the
mistake by not using the library directly in lnd in the first place.
Note that it is still indirectly needed by the btcd secp256k1 library,
therefore the module dependency is still expected to be there, just
moved to the indirect section.
The inclusion proof field in the TapscriptPartialReveal function was
incorrect. An inclusion proof can be zero or more elements of 32-byte
slices. So an empty inclusion proof can be valid too for a tree that
only consists of a single leaf.
