API rename: Sign->SignECDSA, Verify->VerifyECDSA

Summary:
This is being done simply for additional clarity when Schnorr sigs are
introduced in future.

Test Plan: Trivial renames -- it compiles = success.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2345