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_wasm.go

@@ -6,8 +6,6 @@ import "unsafe"
 
 type timeUnit float64 // time in milliseconds, just like Date.now() in JavaScript
 
-const tickMicros = 1000000
-
 // Implements __wasi_ciovec_t and __wasi_iovec_t.
 type wasiIOVec struct {
 	buf    unsafe.Pointer
@@ -68,6 +66,19 @@ func go_scheduler() {
 
 const asyncScheduler = true
 
+func ticksToNanoseconds(ticks timeUnit) int64 {
+	// The JavaScript API works in float64 milliseconds, so convert to
+	// nanoseconds first before converting to a timeUnit (which is a float64),
+	// to avoid precision loss.
+	return int64(ticks * 1e6)
+}
+
+func nanosecondsToTicks(ns int64) timeUnit {
+	// The JavaScript API works in float64 milliseconds, so convert to timeUnit
+	// (which is a float64) first before dividing, to avoid precision loss.
+	return timeUnit(ns) / 1e6
+}
+
 // This function is called by the scheduler.
 // Schedule a call to runtime.scheduler, do not actually sleep.
 //export runtime.sleepTicks