py: Implement iterator support for object that has __getitem__.

Addresses Issue #203.
diff --git a/py/qstrdefs.h b/py/qstrdefs.h
index f2c4dfd..e76efaf 100644
--- a/py/qstrdefs.h
+++ b/py/qstrdefs.h
@@ -13,6 +13,10 @@
 Q(__qualname__)
 Q(__repl_print__)
 
+Q(__getitem__)
+Q(__add__)
+Q(__sub__)
+
 Q(micropython)
 Q(byte_code)
 Q(native)