add nSigChecks counting to EvalScript
Summary:
ref: https://gist.github.com/markblundeberg/becf85ac6ba48bebfeb7871ce2de27a7
Depends on D4798
Test Plan: make check
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Subscribers: Fabien
Maniphest Tasks: T704
Differential Revision: https://reviews.bitcoinabc.org/D4678