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.
This commit adds both positive and negative tests for the new Tx type to
bring its coverage to 100%.
This is part of the ongoing transaction hash optimization effort noted
in conformal/btcd#25.
