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