internal/task: use asyncify on webassembly

This change implements a new "scheduler" for WebAssembly using binaryen's asyncify transform.
This is more reliable than the current "coroutines" transform, and works with non-Go code in the call stack.

runtime (js/wasm): handle scheduler nesting

If WASM calls into JS which calls back into WASM, it is possible for the scheduler to nest.
The event from the callback must be handled immediately, so the task cannot simply be deferred to the outer scheduler.
This creates a minimal scheduler loop which is used to handle such nesting.

src/runtime/scheduler.go

@@ -172,6 +172,24 @@ func scheduler() {
 	}
 }
 
+// This horrible hack exists to make WASM work properly.
+// When a WASM program calls into JS which calls back into WASM, the event with which we called back in needs to be handled before returning.
+// Thus there are two copies of the scheduler running at once.
+// This is a reduced version of the scheduler which does not deal with the timer queue (that is a problem for the outer scheduler).
+func minSched() {
+	scheduleLog("start nested scheduler")
+	for !schedulerDone {
+		t := runqueue.Pop()
+		if t == nil {
+			break
+		}
+
+		scheduleLogTask("  run:", t)
+		t.Resume()
+	}
+	scheduleLog("stop nested scheduler")
+}
+
 func Gosched() {
 	runqueue.Push(task.Current())
 	task.Pause()