Verify arbitrary witness scripts and script-path spends

[aagbotemi]

Summary

Scripts outside the template paths reported inconclusive. They now go to a script interpreter, which verifies taproot script-path spends, hashlocks, timelocks, and single-key witness scripts. Scripts it cannot express still report inconclusive, as the spec permits.

Rebased on #75.

Changes

• Add verify_with_interpreter to evaluate scripts the templates cannot classify.
• Split verify_input into template dispatch and interpreter fallback.
• Add Error::ScriptNotSatisfied for a script that was interpreted but not satisfied.

Notes for reviewers

• Templates must return inconclusive, not an error, when they merely fail to classify a script. An error short-circuits the fallback.

Closes #76.