Table types have system / hidden columns like tableoid, cmax, xmax, etc.
These are not included when sending or receiving composite types.
https://github.com/jackc/pgx/issues/1576
Added ValueRoundTripTest to pgxtest
Removed pgtype/testutil
pgtype tests now run with all (applicable) query modes. This gives
better coverage than before and revealed several bugs which are also
fixed in this commit.