py: Implement builtin reversed() function.

reversed function now implemented, and works for tuple, list, str, bytes
and user objects with __len__ and __getitem__.

Renamed mp_builtin_len to mp_obj_len to make it publically available (eg
for reversed).

py/py.mk

@@ -72,6 +72,7 @@ PY_O_BASENAME = \
 	objnone.o \
 	objnamedtuple.o \
 	objrange.o \
+	objreversed.o \
 	objset.o \
 	objslice.o \
 	objstr.o \