Add a warning about range(enum) iteration since it's really not a good idea...

Add a warning about range(enum) iteration since it's really not a good idea usually, so requiring an explicit cast to say "I know what I'm doing" seems best.

Cython/Compiler/Nodes.py

@@ -6443,6 +6443,9 @@ class ForFromStatNode(LoopNode, StatNode):
         if target_type.is_numeric:
             loop_type = target_type
         else:
+            if target_type.is_enum:
+                warning(self.target.pos,
+                        "Integer loops over enum values are fragile. Please cast to a safe integer type instead.")
             loop_type = PyrexTypes.c_long_type if target_type.is_pyobject else PyrexTypes.c_int_type
             if not self.bound1.type.is_pyobject:
                 loop_type = PyrexTypes.widest_numeric_type(loop_type, self.bound1.type)