py: Define and use MP_OBJ_ITER_BUF_NSLOTS to get size of stack iter buf.

It improves readability of code and reduces the chance to make a mistake.

This patch also fixes a bug with nan-boxing builds by rounding up the
calculation of the new NSLOTS variable, giving the correct number of slots
(being 4) even if mp_obj_t is larger than the native machine size.

py/compile.c

@@ -3005,6 +3005,7 @@ STATIC void compile_scope(compiler_t *comp, scope_t *scope, pass_kind_t pass) {
         // There are 4 slots on the stack for the iterator, and the first one is
         // NULL to indicate that the second one points to the iterator object.
         if (scope->kind == SCOPE_GEN_EXPR) {
+            // TODO static assert that MP_OBJ_ITER_BUF_NSLOTS == 4
             EMIT(load_null);
             compile_load_id(comp, qstr_arg);
             EMIT(load_null);