Implement TaprootBuilder and TaprootSpendInfo

[joemphilips]

This is a prerequisite for #1141 It introduce a logic for building tapscript tree.

• based on types with the same name in rust-bitcoin and bitcoind
  • https://github.com/rust-bitcoin/rust-bitcoin/blob/70eb92cc870b05bd57d5c8d400af8995581fcb9b/bitcoin/src/util/taproot.rs#L184
  • https://github.com/bitcoin/bitcoin/blob/f0c646f026e652082e798800136dc06c734fdab6/src/script/standard.h#L226

It is expected to be used in TransactionBuilder and (possibly in) PSBT.

[joemphilips]

Ready for review. The failing test seems to have nothing to do with my change @NicolasDorier

[NicolasDorier]

Ah I thought it was the taproot output descriptor stuff, but this is actually better. I am happy to get that finally here, I'll use it soon.

[joemphilips]

Descriptor is almost finished too. I've been debugging some nasty bugs related to x-only pubkey.