py/scheduler: Rename sched_stack to sched_queue.

Behaviour was changed from stack to queue in
8977c7eb58, and this updates variable names
to match.  Also updates other references (docs, error messages).

py/modmicropython.c

@@ -150,7 +150,7 @@ STATIC MP_DEFINE_CONST_FUN_OBJ_1(mp_micropython_kbd_intr_obj, mp_micropython_kbd
 #if MICROPY_ENABLE_SCHEDULER
 STATIC mp_obj_t mp_micropython_schedule(mp_obj_t function, mp_obj_t arg) {
     if (!mp_sched_schedule(function, arg)) {
-        mp_raise_msg(&mp_type_RuntimeError, "schedule stack full");
+        mp_raise_msg(&mp_type_RuntimeError, "schedule queue full");
     }
     return mp_const_none;
 }