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