py: Add mp_raise_type helper macro and use it where appropriate.

This provides a more consistent C-level API to raise exceptions, ie moving
away from nlr_raise towards mp_raise_XXX.  It also reduces code size by a
small amount on some ports.

py/modthread.c

@@ -271,7 +271,7 @@ STATIC mp_obj_t mod_thread_start_new_thread(size_t n_args, const mp_obj_t *args)
 STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(mod_thread_start_new_thread_obj, 2, 3, mod_thread_start_new_thread);
 
 STATIC mp_obj_t mod_thread_exit(void) {
-    nlr_raise(mp_obj_new_exception(&mp_type_SystemExit));
+    mp_raise_type(&mp_type_SystemExit);
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_0(mod_thread_exit_obj, mod_thread_exit);