vkFuncDecl, vkFuncPtr, vkClosure; callback example

cl/_testrt/callback/in.go

@@ -0,0 +1,15 @@
+package main
+
+import (
+	"github.com/goplus/llgo/internal/runtime/c"
+)
+
+func callback(f func()) {
+	f()
+}
+
+func main() {
+	callback(func() {
+		c.Printf(c.Str("Hello, callback\n"))
+	})
+}