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