ScanRowValue is useful when reading ROW() values with
known field types as well as composite types. It accepts
pgtype.Value arguments, where ROW() fields are written to
on successfull scan.
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.
Because pgx depends on pgtype and the tests for pgtype depend on pgx
this change will require a couple back and forth commits to get the
go.mod dependecies correct.
Also remove PrepareEx. It's primary usage was for context. Supplying
parameter OIDs is unnecessary when you can type cast in the query SQL.
If it does become necessary or desirable to add options back it can be
added in a backwards compatible way by adding a varargs as last
argument.
Because reading a record type requires the decoder to be able to look up oid
to type mapping and types such as hstore have types that are not fixed between
different PostgreSQL servers it was necessary to restructure the pgtype system
so all encoders and decodes take a *ConnInfo that includes oid/name/type
information.
