all: rewrite sleep function

time.Sleep now compiles on all systems, so lets use that.
Additionally, do a few improvements in time unit handling for the
scheduler. This should lead to somewhat longer sleep durations without
wrapping (on some platforms).

Some examples got smaller, some got bigger. In particular, code using
the scheduler got bigger and the blinky1 example got smaller (especially
on Arduino: 380 -> 314 bytes).

src/runtime/runtime.go

@@ -39,13 +39,6 @@ func main() int {
 	}
 }
 
-func Sleep(d Duration) {
-	// This function is treated specially by the compiler: when goroutines are
-	// used, it is transformed into a llvm.coro.suspend() call.
-	// When goroutines are not used this function behaves as normal.
-	sleep(d)
-}
-
 func GOMAXPROCS(n int) int {
 	// Note: setting GOMAXPROCS is ignored.
 	return 1