feat: optimize insert for class RepeatedComposite. (#95)

Use a standard Python list insert on the target list
to replace a loop using pop and extend.

Co-authored-by: Bob Hancock <rwh@google.com>

proto/marshal/collections/repeated.py

@@ -125,10 +125,4 @@ def __setitem__(self, key, value):
     def insert(self, index: int, value):
         """Insert ``value`` in the sequence before ``index``."""
         pb_value = self._marshal.to_proto(self._pb_type, value, strict=True)
-
-        # Protocol buffers does not define a useful insert, so we have
-        # to pop everything after this point off the list and reload it.
-        after = [pb_value]
-        while self.pb[index:]:
-            after.append(self.pb.pop(index))
-        self.pb.extend(after)
+        self.pb.insert(index, pb_value)