Add support for key-path-spending taproot utxos into transaction.py.
- no wallet support yet
- add some psbt, and minimal descriptor support
- preliminary work towards script-path spends
Instead of some functions operating with hex strings,
and others using bytes, this consolidates most things to use bytes.
This mainly focuses on bitcoin.py and transaction.py,
and then adapts the API usages in other files.
Notably,
- scripts,
- pubkeys,
- signatures
should be bytes in almost all places now.
