script: (refactor) prepare for introducing sighash midstate cache

Summary:
This is a partial backport of core#32473
https://github.com/bitcoin/bitcoin/pull/32473/commits/8f3ddb0bccebc930836b4a6745a7cf29b41eb302

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18848