py/mpstate: Make exceptions thread-local.

This moves mp_pending_exception from mp_state_vm_t to mp_state_thread_t.
This allows exceptions to be scheduled on a specific thread.

Signed-off-by: David Lechner <david@pybricks.com>

py/runtime.c

@@ -62,7 +62,7 @@ void mp_init(void) {
     qstr_init();
 
     // no pending exceptions to start with
-    MP_STATE_VM(mp_pending_exception) = MP_OBJ_NULL;
+    MP_STATE_THREAD(mp_pending_exception) = MP_OBJ_NULL;
     #if MICROPY_ENABLE_SCHEDULER
     MP_STATE_VM(sched_state) = MP_SCHED_IDLE;
     MP_STATE_VM(sched_idx) = 0;