Renderers: Make appending rows massively more efficient

Previously we were generating the list of children (for most likely the
root node) for every single append statement, during which we were
recalculating the length of the list, twice.  In plugins that output a
lot of rows this would add an enourmous overhead (that likely grew as
the length of the output grew).  Without this overhead, the time taken
for the TreeGrid._append method went from 1230.0s to 2.4s.

volatility3/framework/renderers/__init__.py

@@ -272,20 +272,26 @@ class TreeGrid(interfaces.renderers.TreeGrid):
     def _append(self, parent: Optional[interfaces.renderers.TreeNode], values: Any) -> TreeNode:
         """Adds a new node at the top level if parent is None, or under the
         parent node otherwise, after all other children."""
-        children = self.children(parent)
-        return self._insert(parent, len(children), values)
+        return self._insert(parent, None, values)
 
-    def _insert(self, parent: Optional[interfaces.renderers.TreeNode], position: int, values: Any) -> TreeNode:
+    def _insert(self, parent: Optional[interfaces.renderers.TreeNode], position: Optional[int], values: Any) -> TreeNode:
         """Inserts an element into the tree at a specific position."""
         parent_path = ""
         children = self._find_children(parent)
         if parent is not None:
             parent_path = parent.path + self.path_sep
-        newpath = parent_path + str(position)
+        if position is None:
+            newpath = parent_path + str(len(children))
+        else:
+            newpath = parent_path + str(position)
+            for node, _ in children[position:]:
+                self.visit(node, lambda child, _: child.path_changed(newpath, True), None)
+
         tree_item = TreeNode(newpath, self, parent, values)
-        for node, _ in children[position:]:
-            self.visit(node, lambda child, _: child.path_changed(newpath, True), None)
-        children.insert(position, (tree_item, []))
+        if position is None:
+            children.append((tree_item, []))
+        else:
+            children.insert(position, (tree_item, []))
         return tree_item
 
     def is_ancestor(self, node, descendant):