py: Add iter_buf to getiter type method.

Allows to iterate over the following without allocating on the heap:
- tuple
- list
- string, bytes
- bytearray, array
- dict (not dict.keys, dict.values, dict.items)
- set, frozenset

Allows to call the following without heap memory:
- all, any, min, max, sum

TODO: still need to allocate stack memory in bytecode for iter_buf.

py/objreversed.c

@@ -74,7 +74,7 @@ const mp_obj_type_t mp_type_reversed = {
     { &mp_type_type },
     .name = MP_QSTR_reversed,
     .make_new = reversed_make_new,
-    .getiter = mp_identity,
+    .getiter = mp_identity_getiter,
     .iternext = reversed_iternext,
 };