Add an utility method to check if a script is a comitment to a certain value.

Summary: This will help implement anti replay feature.

Test Plan: I added test cases for this method in script_tests.cpp

Reviewers: freetrader, sickpig, kyuupichan, #bitcoin_abc

Reviewed By: freetrader, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D147