runtime: refactor time handling

This commit refactors both determining the current time and sleeping for
a given time. It also improves precision for many chips.

  * The nrf chips had a long-standing TODO comment about a slightly
    inaccurate clock. This should now be fixed.
  * The SAM D2x/D5x chips may have a slightly more accurate clock,
    although probably within the error margin of the RTC. Also, by
    working with RTC ticks and converting in the least number of places,
    code size is often slightly reduced (usually just a few bytes, up to
    around 1kB in some cases).
  * I believe the HiFive1 rev B timer was slightly wrong (32768Hz vs
    30517.6Hz). Because the datasheet says the clock runs at 32768Hz,
    I've used the same conversion code here as in the nrf and sam cases.
  * I couldn't test both stm32 timers, so I kept them as they currently
    are. It may be possible to make them more efficient by using the
    native tick frequency instead of using microseconds everywhere.

src/runtime/runtime_unix.go

@@ -26,8 +26,6 @@ func clock_gettime(clk_id int32, ts *timespec)
 
 type timeUnit int64
 
-const tickMicros = 1
-
 // Note: tv_sec and tv_nsec vary in size by platform. They are 32-bit on 32-bit
 // systems and 64-bit on 64-bit systems (at least on macOS/Linux), so we can
 // simply use the 'int' type which does the same.
@@ -57,7 +55,18 @@ func putchar(c byte) {
 
 const asyncScheduler = false
 
+func ticksToNanoseconds(ticks timeUnit) int64 {
+	// The OS API works in nanoseconds so no conversion necessary.
+	return int64(ticks)
+}
+
+func nanosecondsToTicks(ns int64) timeUnit {
+	// The OS API works in nanoseconds so no conversion necessary.
+	return timeUnit(ns)
+}
+
 func sleepTicks(d timeUnit) {
+	// timeUnit is in nanoseconds, so need to convert to microseconds here.
 	usleep(uint(d) / 1000)
 }