Set a ValueTranscoder from the same type errors

[jschaf]

I'm trying to generate code to handle a composite type that contains a pgtype.Interval: https://github.com/jschaf/pggen/issues/49. The main issue seems to be that pgtype doesn't support Set with the same type. This test case shows the error:

func TestPgtype_Interval(t *testing.T) {
	v1 := pgtype.Interval{Days: 2, Status: pgtype.Present}
	v2 := pgtype.Interval{Days: 2, Status: pgtype.Present}
	err := v1.Set(v2)
	require.NoError(t, err) // Received unexpected error: cannot convert {0 2 0 2} to Interval
}

Two questions:

1. Is it reasonable to support Set from the same type? Get returns the same type so it'd have a nice symmetry.
2. Would you take a PR for this feature?

[jackc]

Seems reasonable.