An address is conceptually a human-readable string encoding of a
payment script, so the two methods String and PaymentScript should be
sufficient to provide basic address functionality. This also avoids
some stutter/repetition (e.g. ma.Address().Address()) in wallet code,
where wrapper types are used around the Address interface, and
additionally allows the wrapper types to implement the stdaddr.Address
interface directly while still providing their own Address method to
return the underlying interface type.
This moves the new stdaddr package from the internal staging area to the
txscript module and updates the relevant paths and package README.md
accordingly.
