perf: implement append to avoid invalidating id2offset (#94)

docarray/array/storage/memory/seqlike.py

@@ -17,6 +17,15 @@ def insert(self, index: int, value: 'Document'):
         """
         self._data.insert(index, value)
 
+    def append(self, value: 'Document'):
+        """Append `doc` to the end of the array.
+
+        :param value: The doc needs to be appended.
+        """
+        self._data.append(value)
+        if not self._needs_id2offset_rebuild:
+            self._id_to_index[value.id] = len(self) - 1
+
     def __eq__(self, other):
         return (
             type(self) is type(other)