6715e93e89 Add Witness::p2tr_key_spend function (Tobin C. Harding)
Pull request description:
Add a function for creating the witness when doing a key path spend for a P2TR output.
This mirrors what we did for P2WPKH when adding `Witness::p2wpkh`.
Includes update to the taproot signing example to use the new constructor.
ACKs for top commit:
Kixunil:
ACK 6715e93e89
apoelstra:
ACK 6715e93e89
Tree-SHA512: aab51329e8fda471442bb9cebd6327636548dd157bb9842fe66993fcdd211bb04b2b829aa9d5962dd619f5c0b73d19644a44529c1a5958df1a6bc892147b44f5