Huge refactoring for C++ and low-power mode

The display currently shows the time, with hours and minutes, and is
capable of receiving input with buttons (though does nothing). It
sleeps during intervals where nothing is happening. The display task
runs once per second, and RTC alarm A is used for periodic alarms to
update the system time.

Time.h

@@ -57,6 +57,26 @@ public:
     {
         return value * MICROS_PER_SEC;
     }
+
+    static inline uint64_t to_nanos(time_t value)
+    {
+        return value * NANOS_PER_MICRO;
+    }
+
+    static inline uint64_t to_micros(time_t value)
+    {
+        return value;
+    }
+
+    static inline uint64_t to_millis(time_t value)
+    {
+        return value / MICROS_PER_MILLI;
+    }
+
+    static inline uint64_t to_seconds(time_t value)
+    {
+        return value / MICROS_PER_SEC;
+    }
 };
 
 }