[ecash-lib] Add signRecoverable and recoverSig to Ecc

Summary:
Now that the HD wallet is also in ecash-lib, if we can get msg signing, then CashTab could lose bitgo/utxo-lib and bitcoinjs-message as deps.

We use our secp256k1, using Pieter Wuille's blessed code, which is much more rigorously vetted than bitcoinjs-message.

Test Plan: npm test

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D17667