Rather than special case the reported issue with UUID and [16]byte, this
commit allows the system to find the underlying type of any type that is
an array.
fixes https://github.com/jackc/pgx/issues/2107
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.