Prefer driver.Value over wrap plans when encoding

This is tricky due to driver.Valuer returning any. For example, we can
plan for fmt.Stringer because it always returns a string.

Because of this driver.Valuer was always handled as the last option. But
with pgx v5 now having the ability to find underlying types like a
string and supporting fmt.Stringer it meant that driver.Valuer was
often not getting called because something else was found first.

This change tries driver.Valuer immediately after the initial PlanScan
for the Codec. So a type that directly implements a pgx interface should
be used, but driver.Valuer will be prefered before all the attempts to
handle renamed types, pointer deferencing, etc.

fixes https://github.com/jackc/pgx/issues/1319
fixes https://github.com/jackc/pgx/issues/1311

pgtype/pgtype_test.go

@@ -3,7 +3,9 @@ package pgtype_test
 import (
 	"context"
 	"database/sql"
+	"database/sql/driver"
 	"errors"
+	"fmt"
 	"net"
 	"os"
 	"regexp"
@@ -265,6 +267,56 @@ func TestMapScanPtrToPtrToSlice(t *testing.T) {
 	require.Equal(t, []string{"foo", "bar"}, *v)
 }
 
+type databaseValuerString string
+
+func (s databaseValuerString) Value() (driver.Value, error) {
+	return fmt.Sprintf("%d", len(s)), nil
+}
+
+// https://github.com/jackc/pgx/issues/1319
+func TestMapEncodeTextFormatDatabaseValuerThatIsRenamedSimpleType(t *testing.T) {
+	m := pgtype.NewMap()
+	src := databaseValuerString("foo")
+	buf, err := m.Encode(pgtype.TextOID, pgtype.TextFormatCode, src, nil)
+	require.NoError(t, err)
+	require.Equal(t, "3", string(buf))
+}
+
+type databaseValuerFmtStringer string
+
+func (s databaseValuerFmtStringer) Value() (driver.Value, error) {
+	return nil, nil
+}
+
+func (s databaseValuerFmtStringer) String() string {
+	return "foobar"
+}
+
+// https://github.com/jackc/pgx/issues/1311
+func TestMapEncodeTextFormatDatabaseValuerThatIsFmtStringer(t *testing.T) {
+	m := pgtype.NewMap()
+	src := databaseValuerFmtStringer("")
+	buf, err := m.Encode(pgtype.TextOID, pgtype.TextFormatCode, src, nil)
+	require.NoError(t, err)
+	require.Nil(t, buf)
+}
+
+type databaseValuerStringFormat struct {
+	n int32
+}
+
+func (v databaseValuerStringFormat) Value() (driver.Value, error) {
+	return fmt.Sprint(v.n), nil
+}
+
+func TestMapEncodeBinaryFormatDatabaseValuerThatReturnsString(t *testing.T) {
+	m := pgtype.NewMap()
+	src := databaseValuerStringFormat{n: 42}
+	buf, err := m.Encode(pgtype.Int4OID, pgtype.BinaryFormatCode, src, nil)
+	require.NoError(t, err)
+	require.Equal(t, []byte{0, 0, 0, 42}, buf)
+}
+
 func BenchmarkMapScanInt4IntoBinaryDecoder(b *testing.B) {
 	m := pgtype.NewMap()
 	src := []byte{0, 0, 0, 42}