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_avr.go

@@ -14,8 +14,6 @@ type timeUnit uint32
 
 var currentTime timeUnit
 
-const tickMicros = 1024 * 16384
-
 // Watchdog timer periods. These can be off by a large margin (hence the jump
 // between 64ms and 125ms which is not an exact double), so don't rely on this
 // for accurate time keeping.
@@ -71,6 +69,16 @@ func putchar(c byte) {
 
 const asyncScheduler = false
 
+const tickNanos = 1024 * 16384 // roughly 16ms in nanoseconds
+
+func ticksToNanoseconds(ticks timeUnit) int64 {
+	return int64(ticks) * tickNanos
+}
+
+func nanosecondsToTicks(ns int64) timeUnit {
+	return timeUnit(ns / tickNanos)
+}
+
 // Sleep this number of ticks of 16ms.
 //
 // TODO: not very accurate. Improve accuracy by calibrating on startup and every