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/emitnative.c

@@ -1806,6 +1806,7 @@ STATIC void emit_native_get_iter(emit_t *emit) {
     vtype_kind_t vtype;
     emit_pre_pop_reg(emit, &vtype, REG_ARG_1);
     assert(vtype == VTYPE_PYOBJ);
+    assert(0); // TODO allocate memory for iter_buf
     emit_call(emit, MP_F_GETITER);
     emit_post_push_reg(emit, VTYPE_PYOBJ, REG_RET);
 }