Add Hash160 func to calculate ripemd160(sha256(data)).
Implement Address interface.

Address is a generic interface for any type of "address" a
transaction can be sent to, including but not limited to
pay-to-pubkey, pay-to-pubkey-hash, and pay-to-script-hash.

This change implements Address and concrete types for P2PKH and P2SH
addresses with 100% test coverage.  Pay-to-pubkey support will be
added in the future.

This API is intended to replace the old EncodeAddress/DecodeAddress
functions which are now deprecated.