make CheckMinimalPush available to codebase

Summary:
Besides the script evaluator, it can be useful in other script parsers to
find out whether a given push was minimal or not.

Test Plan: make check

Reviewers: jasonbcox, Fabien, deadalnix, #bitcoin_abc

Reviewed By: jasonbcox, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3872