Taproot support

[MatthewLM]

In rough order of priority.

Basic initial key-path spending:

• 7

• 15

• 8

• 6

• 9

• 10

• #11

• 12

• 13

Script-path spending:

• 14

• 16

• 17

• 18

[MatthewLM]

Completed all the issues. Requires:

• [x] A successful script-path testnet transaction.
• [x] Checks for test coverage.
• [x] PR with code review.