Taproot tracking issue

As of the 0.21 rebase we have Taproot in Elements. There is some folowup work we need to make this usable

- [x] update sighash algorithm (see #933 for design doc)
- [x] add new opcodes to Tapscript (design doc forthcoming)
- [x] update CHECKSIGFROMSTACK to support variable-length messages
- [ ] SIGHASH_NOINPUT?
- [ ] add Taproot to dynafed headers
- [ ] scope out pre-dynafed Taproot in blockheaders; can we softfork it and how much work would this be?
- [ ] add Taproot descriptors (related: descriptors tracking issue https://github.com/ElementsProject/elements/issues/944)
- [ ] add Taproot fields to PSET (needs own design doc / coordination with Bitcoin)
- [ ] add Simplicity as a Tapleaf (related: simplicity tracking issue #945)