Add pseudo-get delay function for scheduled functions

cores/esp8266/Schedule.cpp

@@ -149,6 +149,11 @@ uint32_t get_scheduled_recurrent_delay_us()
     return (remaining > 0) ? static_cast<uint32_t>(remaining) : 0;
 }
 
+uint32_t get_scheduled_delay_us()
+{
+    return sFirst ? 0 : ~static_cast<decltype(micros())>(0) >> 1;
+}
+
 void run_scheduled_functions()
 {
     // prevent scheduling of new functions during this run

cores/esp8266/Schedule.h

@@ -60,6 +60,13 @@ uint32_t get_scheduled_recurrent_delay_us();
 // * Run the lambda only once next time.
 // * A scheduled function can schedule a function.
 
+// get_scheduled_delay_us() is named for symmetry to get_scheduled_recurrent_delay_us,
+// despite the lack of specific delay times. Therefore it can return only one of two
+// values, viz. 0 in case of any pending scheduled functions, or a large delay time if
+// there is no function in the queue.
+
+uint32_t get_scheduled_delay_us();
+
 bool schedule_function (const std::function<void(void)>& fn);
 
 // Run all scheduled functions.