Move function checking encoding of public key and signature out of interpreter.{h/cpp}
Summary: This improves modularity and will allow for more targeted testing.
Test Plan:
make check
Reviewers: #bitcoin_abc, Mengerian, jasonbcox
Reviewed By: #bitcoin_abc, Mengerian, jasonbcox
Subscribers: Mengerian, teamcity
Differential Revision: https://reviews.bitcoinabc.org/D1565