reflect/protoreflect: add Descriptor specific methods

Added methods:
	Enum.Descriptor
	Message.Descriptor
	EnumType.Descriptor
	MessageType.Descriptor
	ExtensionType.Descriptor
	Message.New

All functionality is switched over to use those methods instead of
implicitly relying on the fact that {Enum,Message}Type implicitly
implement the associated descriptor interface.

This CL does not yet remove {Enum,Message}.Type or prevent
{Enum,Message,Extension}Type from implementating a descriptor.
That is a subsequent CL.

The Message.New method is also added to replace functionality
that will be lost when the Type methods are removed.

Change-Id: I7fefde1673bbd40bfdac489aca05cec9a6c98eb1
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/174918
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Herbie Ong <herbie@google.com>
diff --git a/proto/size.go b/proto/size.go
index 3bc5a3c..b881653 100644
--- a/proto/size.go
+++ b/proto/size.go
@@ -34,14 +34,14 @@
 }
 
 func sizeMessage(m protoreflect.Message) (size int) {
-	fields := m.Type().Fields()
+	fieldDescs := m.Descriptor().Fields()
 	knownFields := m.KnownFields()
 	m.KnownFields().Range(func(num protoreflect.FieldNumber, value protoreflect.Value) bool {
-		field := fields.ByNumber(num)
+		field := fieldDescs.ByNumber(num)
 		if field == nil {
-			field = knownFields.ExtensionTypes().ByNumber(num)
+			field = knownFields.ExtensionTypes().ByNumber(num).Descriptor()
 			if field == nil {
-				panic(fmt.Errorf("no descriptor for field %d in %q", num, m.Type().FullName()))
+				panic(fmt.Errorf("no descriptor for field %d in %q", num, m.Descriptor().FullName()))
 			}
 		}
 		size += sizeField(field, value)