compiler,runtime: implement stack-based scheduler

This scheduler is intended to live along the (stackless) coroutine based
scheduler which is needed for WebAssembly and unsupported platforms. The
stack based scheduler is somewhat simpler in implementation as it does
not require full program transform passes and supports things like
function pointers and interface methods out of the box with no changes.

Code size is reduced in most cases, even in the case where no scheduler
scheduler is used at all. I'm not exactly sure why but these changes
likely allowed some further optimizations somewhere. Even RAM is
slightly reduced, perhaps some global was elminated in the process as
well.

src/runtime/runtime.go

@@ -79,11 +79,6 @@ func memequal(x, y unsafe.Pointer, n uintptr) bool {
 	return true
 }
 
-//go:linkname sleep time.Sleep
-func sleep(d int64) {
-	sleepTicks(timeUnit(d / tickMicros))
-}
-
 func nanotime() int64 {
 	return int64(ticks()) * tickMicros
 }