This adds a --dumptests flag to the feature_taproot.py test, to dump all its
generated test cases to files, in a format compatible with the
script_assets_test unit test. A fuzzer for said format is added as well, whose
primary purpose is coverage-based minimization of those dumps.
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.
