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/objgetitemiter.c

@@ -61,13 +61,14 @@ STATIC mp_obj_t it_iternext(mp_obj_t self_in) {
 STATIC const mp_obj_type_t it_type = {
     { &mp_type_type },
     .name = MP_QSTR_iterator,
-    .getiter = mp_identity,
+    .getiter = mp_identity_getiter,
     .iternext = it_iternext,
 };
 
 // args are those returned from mp_load_method_maybe (ie either an attribute or a method)
-mp_obj_t mp_obj_new_getitem_iter(mp_obj_t *args) {
-    mp_obj_getitem_iter_t *o = m_new_obj(mp_obj_getitem_iter_t);
+mp_obj_t mp_obj_new_getitem_iter(mp_obj_t *args, mp_obj_iter_buf_t *iter_buf) {
+    assert(sizeof(mp_obj_getitem_iter_t) <= sizeof(mp_obj_iter_buf_t));
+    mp_obj_getitem_iter_t *o = (mp_obj_getitem_iter_t*)iter_buf;
     o->base.type = &it_type;
     o->args[0] = args[0];
     o->args[1] = args[1];