tests: functional tests for Schnorr/Taproot/Tapscript

A large functional test is added that automatically generates random transactions which
exercise various aspects of the new rules, and verifies they are accepted into the mempool
(when appropriate), and correctly accepted/rejected in (Python-constructed) blocks.

Includes sighashing code and many tests by Johnson Lau.
Includes a test by Matthew Zipkin.
Includes several tests and improvements by Greg Sanders.

Author: sipa