Add CompositeFields type

This adds support for the text format and removes the need for the
ScanRowValue function.

composite_fields.go

@@ -0,0 +1,76 @@
+package pgtype
+
+import (
+	errors "golang.org/x/xerrors"
+)
+
+// CompositeFields scans the fields of a composite type into the elements of the CompositeFields value. To scan a
+// nullable value use a *CompositeFields. It will be set to nil in case of null.
+type CompositeFields []interface{}
+
+func (cf CompositeFields) DecodeBinary(ci *ConnInfo, src []byte) error {
+	if len(cf) == 0 {
+		return errors.Errorf("cannot decode into empty CompositeFields")
+	}
+
+	if src == nil {
+		return errors.Errorf("cannot decode unexpected null into CompositeFields")
+	}
+
+	scanner, err := NewCompositeBinaryScanner(src)
+	if err != nil {
+		return err
+	}
+	if len(cf) != scanner.FieldCount() {
+		return errors.Errorf("SQL composite can't be read, field count mismatch. expected %d , found %d", len(cf), scanner.FieldCount())
+	}
+
+	for i := 0; scanner.Scan(); i++ {
+		err := ci.Scan(scanner.OID(), BinaryFormatCode, scanner.Bytes(), cf[i])
+		if err != nil {
+			return err
+		}
+	}
+
+	if scanner.Err() != nil {
+		return scanner.Err()
+	}
+
+	return nil
+}
+
+func (cf CompositeFields) DecodeText(ci *ConnInfo, src []byte) error {
+	if len(cf) == 0 {
+		return errors.Errorf("cannot decode into empty CompositeFields")
+	}
+
+	if src == nil {
+		return errors.Errorf("cannot decode unexpected null into CompositeFields")
+	}
+
+	scanner, err := NewCompositeTextScanner(src)
+	if err != nil {
+		return err
+	}
+
+	fieldCount := 0
+
+	for i := 0; scanner.Scan(); i++ {
+		err := ci.Scan(0, TextFormatCode, scanner.Bytes(), cf[i])
+		if err != nil {
+			return err
+		}
+
+		fieldCount += 1
+	}
+
+	if scanner.Err() != nil {
+		return scanner.Err()
+	}
+
+	if len(cf) != fieldCount {
+		return errors.Errorf("SQL composite can't be read, field count mismatch. expected %d , found %d", len(cf), fieldCount)
+	}
+
+	return nil
+}