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/objfilter.c

@@ -39,7 +39,7 @@ STATIC mp_obj_t filter_make_new(const mp_obj_type_t *type, size_t n_args, size_t
     mp_obj_filter_t *o = m_new_obj(mp_obj_filter_t);
     o->base.type = type;
     o->fun = args[0];
-    o->iter = mp_getiter(args[1]);
+    o->iter = mp_getiter(args[1], NULL);
     return MP_OBJ_FROM_PTR(o);
 }
 
@@ -65,7 +65,7 @@ const mp_obj_type_t mp_type_filter = {
     { &mp_type_type },
     .name = MP_QSTR_filter,
     .make_new = filter_make_new,
-    .getiter = mp_identity,
+    .getiter = mp_identity_getiter,
     .iternext = filter_iternext,
 };