rename files to easily grouping

Author: Mari Wahl

src/builtin_structures/find_if_is_substr.py renamed to src/builtin_structures/find_if_substr.py

@@ -26,14 +26,15 @@ def _add(self, value):
             self.right = new_node
         else:
             self.left = self.left._add(value)
+
         return self
 
 
     def _search(self, value):
         if self.item == value:
             return True # or self
 
-        found = False # or False, thats diff from BST
+        found = False # or None, thats diff from BST
         if self.left:
             found = self.left._search(value)