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.
diff --git a/py/objzip.c b/py/objzip.c
index 6edefc3..5d9ba48 100644
--- a/py/objzip.c
+++ b/py/objzip.c
@@ -43,7 +43,7 @@
     o->base.type = type;
     o->n_iters = n_args;
     for (mp_uint_t i = 0; i < n_args; i++) {
-        o->iters[i] = mp_getiter(args[i]);
+        o->iters[i] = mp_getiter(args[i], NULL);
     }
     return MP_OBJ_FROM_PTR(o);
 }
@@ -71,6 +71,6 @@
     { &mp_type_type },
     .name = MP_QSTR_zip,
     .make_new = zip_make_new,
-    .getiter = mp_identity,
+    .getiter = mp_identity_getiter,
     .iternext = zip_iternext,
 };