[test_framework] Move sighash related functions to signature_hash.py

Summary:
These are a bit out of place in script.py. This makes script.py independent of the rest of the framework so we can use it in the plugin system.

Depends on D16549.

Test Plan: ninja check-functional

Reviewers: Fabien, #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16550