Update github pages demo for go 1.12+

- go 1.12 optimizes memory allocation for wasm - see
  golang/go#27462. This was causing the demo to OOM
  on memory restricted devices (e.g. phones).
- js namespace changed so we need to adapt to that

Author: niklasfasching

etc/gh-pages/_js.go

@@ -16,22 +16,23 @@ func main() {
 	document := js.Global().Get("document")
 	in := document.Call("getElementById", "gowen-input")
 	out := document.Call("getElementById", "gowen-output")
-	js.Global().Set("gowenRun", js.NewCallback(func([]js.Value) {
+	js.Global().Set("gowenRun", js.FuncOf(func(js.Value, []js.Value) interface{} {
 		nodes, err := gowen.Parse(in.Get("value").String())
 		if err != nil {
 			out.Set("textContent", fmt.Sprintf("%s", err))
-			return
+			return nil
 		}
 		results, err := gowen.EvalMultiple(nodes, env)
 		if err != nil {
 			out.Set("textContent", fmt.Sprintf("%s", err))
-			return
+			return nil
 		}
 		s := ""
 		for _, result := range results {
 			s += pretty.Sprint(result.ToGo()) + "\n"
 		}
 		out.Set("textContent", s)
+		return nil
 	}))
 
 	<-make(chan struct{}) // stay alive