[ecash-lib] Add ecdsaVerify and schnorrVerify to Ecc

Summary: Allows verifying ECDSA and our Schnorr signatures. Argument order is the same as for OP_CHECKDATASIG.

Test Plan: npm test

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D17732