secp256k1: Decouple signing from ecdsa.PrivateKey.
secp256k1: Decouple signing from ecdsa.PrivateKey.

This is continued work towards making the internals of the package
entirely independent of the crypto/elliptic and crypto/ecdsa stdlib
interfaces.

decred/dcrd,jrick/dcrd,decred/dcrd,jrick/dcrd