Find child id once

Rather than computing the child id for every iteration of walking
backwards, we do it once.

libsaria/trees.py

@@ -55,7 +55,8 @@ class DLTree(Tree):
 	def walk_backwards(self, child = None):
 		key = None
 		if child != None:
-			key = [item[0] for item in self.items() if id(item[1]) == id(child)]
+			idc = id(child)
+			key = [i[0] for i in self.items() if id(i[1]) == idc]
 			if self.parent == None:
 				return key
 		ret = self.parent.walk_backwards(self)