Increased data type support for simple protocol. Improved test
coverage of simple protocol. This has the additional advantage of
exercising the text encoders and decoders.
1. arg is a pointer to nil
2. The type of arg implements BinaryEncoder or TextEncoder
3. The implementation of that interface uses a non-pointer receiver
It was a mistake to use it in other contexts. This made interop
difficult between pacakges that depended on pgtype such as pgx and
packages that did not like pgconn and pgproto3. In particular this was
awkward for prepared statements.
This is preparation for removing pgx.PreparedStatement in favor of
pgconn.PreparedStatement.
