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/objenumerate.c

@@ -56,13 +56,13 @@ STATIC mp_obj_t enumerate_make_new(const mp_obj_type_t *type, size_t n_args, siz
     // create enumerate object
     mp_obj_enumerate_t *o = m_new_obj(mp_obj_enumerate_t);
     o->base.type = type;
-    o->iter = mp_getiter(arg_vals.iterable.u_obj);
+    o->iter = mp_getiter(arg_vals.iterable.u_obj, NULL);
     o->cur = arg_vals.start.u_int;
 #else
     (void)n_kw;
     mp_obj_enumerate_t *o = m_new_obj(mp_obj_enumerate_t);
     o->base.type = type;
-    o->iter = mp_getiter(args[0]);
+    o->iter = mp_getiter(args[0], NULL);
     o->cur = n_args > 1 ? mp_obj_get_int(args[1]) : 0;
 #endif
 
@@ -74,7 +74,7 @@ const mp_obj_type_t mp_type_enumerate = {
     .name = MP_QSTR_enumerate,
     .make_new = enumerate_make_new,
     .iternext = enumerate_iternext,
-    .getiter = mp_identity,
+    .getiter = mp_identity_getiter,
 };
 
 STATIC mp_obj_t enumerate_iternext(mp_obj_t self_in) {