esp32: Reduce latency for handling of scheduled Python callbacks.

Prior to this patch there was a large latency for executing scheduled
callbacks when when Python code is sleeping: at the heart of the
implementation of sleep_ms() is a call to vTaskDelay(1), which always
sleeps for one 100Hz tick, before performing another call to
MICROPY_EVENT_POLL_HOOK.

This patch fixes this issue by using FreeRTOS Task Notifications to signal
the main thread that a new callback is pending.

ports/esp32/machine_pin.c

@@ -33,6 +33,7 @@
 
 #include "py/runtime.h"
 #include "py/mphal.h"
+#include "mphalport.h"
 #include "modmachine.h"
 #include "extmod/virtpin.h"
 #include "machine_rtc.h"
@@ -115,6 +116,7 @@ STATIC void IRAM_ATTR machine_pin_isr_handler(void *arg) {
     machine_pin_obj_t *self = arg;
     mp_obj_t handler = MP_STATE_PORT(machine_pin_irq_handler)[self->id];
     mp_sched_schedule(handler, MP_OBJ_FROM_PTR(self));
+    mp_hal_wake_main_task_from_isr();
 }
 
 gpio_num_t machine_pin_get_id(mp_obj_t pin_in) {