callback: handle more cases

this fails to compile. Currently this is commented out in the examples/cb

```
#[node_bindgen]
fn sum String>(cb: F,second: i32) -> String {
let message = cb(second*2);
format!("my message: {}",message)
}
```

Start by reproducing the compile failure from examples/cb using the shown generic callback signature and capture the compiler diagnostics. Trace the callback handling used by node_bindgen, then verify that the example compiles and the callback case works.

Written by the indexing model from the issue text.