Avoid type assertion in Scan

Before:
BenchmarkConnInfoScanInt4IntoBinaryDecoder-16           79859755                14.6 ns/op             0 B/op          0 allocs/op
BenchmarkConnInfoScanInt4IntoGoInt32-16                 38969991                30.0 ns/op             0 B/op          0 allocs/op

After:
BenchmarkConnInfoScanInt4IntoBinaryDecoder-16    	458046958	        13.3 ns/op	       0 B/op	       0 allocs/op
BenchmarkConnInfoScanInt4IntoGoInt32-16          	275791776	        20.6 ns/op	       0 B/op	       0 allocs/op

pgtype.go

@@ -164,8 +164,12 @@ var errBadStatus = errors.New("invalid status")
 
 type DataType struct {
 	Value Value
-	Name  string
+
-	OID   uint32
+	textDecoder   TextDecoder
+	binaryDecoder BinaryDecoder
+
+	Name string
+	OID  uint32
 }
 
 type ConnInfo struct {
@@ -285,6 +289,14 @@ func (ci *ConnInfo) RegisterDataType(t DataType) {
 		}
 		ci.oidToResultFormatCode[t.OID] = formatCode
 	}
+
+	if d, ok := t.Value.(TextDecoder); ok {
+		t.textDecoder = d
+	}
+
+	if d, ok := t.Value.(BinaryDecoder); ok {
+		t.binaryDecoder = d
+	}
 }
 
 func (ci *ConnInfo) DataTypeForOID(oid uint32) (*DataType, bool) {
@@ -374,25 +386,24 @@ func (ci *ConnInfo) Scan(oid uint32, formatCode int16, buf []byte, dest interfac
 	}
 
 	if dt, ok := ci.DataTypeForOID(oid); ok {
-		value := dt.Value
 		switch formatCode {
-		case TextFormatCode:
-			if textDecoder, ok := value.(TextDecoder); ok {
-				err := textDecoder.DecodeText(ci, buf)
-				if err != nil {
-					return err
-				}
-			} else {
-				return errors.Errorf("%T is not a pgtype.TextDecoder", value)
-			}
 		case BinaryFormatCode:
-			if binaryDecoder, ok := value.(BinaryDecoder); ok {
+			if dt.binaryDecoder != nil {
-				err := binaryDecoder.DecodeBinary(ci, buf)
+				err := dt.binaryDecoder.DecodeBinary(ci, buf)
 				if err != nil {
 					return err
 				}
 			} else {
-				return errors.Errorf("%T is not a pgtype.BinaryDecoder", value)
+				return errors.Errorf("%T is not a pgtype.BinaryDecoder", dt.Value)
+			}
+		case TextFormatCode:
+			if dt.textDecoder != nil {
+				err := dt.textDecoder.DecodeText(ci, buf)
+				if err != nil {
+					return err
+				}
+			} else {
+				return errors.Errorf("%T is not a pgtype.TextDecoder", dt.Value)
 			}
 		default:
 			return errors.Errorf("unknown format code: %v", formatCode)
@@ -400,7 +411,7 @@ func (ci *ConnInfo) Scan(oid uint32, formatCode int16, buf []byte, dest interfac
 
 		if !isFastType {
 			if scanner, ok := dest.(sql.Scanner); ok {
-				sqlSrc, err := DatabaseSQLValue(ci, value)
+				sqlSrc, err := DatabaseSQLValue(ci, dt.Value)
 				if err != nil {
 					return err
 				}
@@ -408,7 +419,7 @@ func (ci *ConnInfo) Scan(oid uint32, formatCode int16, buf []byte, dest interfac
 			}
 		}
 
-		return value.AssignTo(dest)
+		return dt.Value.AssignTo(dest)
 	}
 
 	// We might be given a pointer to something that implements the decoder interface(s),