Rather than repeating the same code in both the pay-to-pubkey-hash and
pay-to-script-hash encoding, refactor it into separate functions. This
makes it easier to support new networks, for example, since it can be
changed in one place.
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.
