Allow moving NativeObject variables into closures as external captures

[jedel1043]

This Pull Request follows from #1518.

It changes the following:

• Allows moving any NativeObject + Clone type as a parameter into any closure
• Adds test for the new feature

This unblocks the implementation of arguments exotic objects.
The new test is a good example of how this new feature works:

let js_function = FunctionBuilder::closure_with_captures(
        &mut context,
        |_, _, context, captures| {
            println!("Called `createMessage`");
            // We obtain the `name` property of `captures.object`
            let name = captures.object.get("name", context)?;

            // We create a new message from our captured variable.
            let message = JsString::concat_array(&[
                "message from `",
                name.to_string(context)?.as_str(),
                "`: ",
                captures.greeting.as_str(),
            ]);

            println!("{}", message);

            // We convert `message` into `Jsvalue` to be able to return it.
            Ok(message.into())
        },
        // Here is where we move `clone_variable` into the closure.
        clone_variable,
    )

[raskad]

Looks good to me.
Maybe you could add the test (or something similar) to the examples? NativeObject is pretty abstract and it may not be clear what you can do with closure_with_captures without an example.

[jedel1043]

Looks good to me.
Maybe you could add the test (or something similar) to the examples? NativeObject is pretty abstract and it may not be clear what you can do with closure_with_captures without an example.

Okay, let me add an example

[jedel1043]

Looks good to me.
Maybe you could add the test (or something similar) to the examples? NativeObject is pretty abstract and it may not be clear what you can do with closure_with_captures without an example.

Added an example inside closures.rs. Also slightly modified the API to automatically downcast to the passed type. Let me know if you like the new changes.

[raskad]

Wow, the automatic downcast looks really nice from an API consumer perspective. We only have to make sure that the downcast never fails, otherwise the error is really confusing.

The example looks great!

[raskad]

I just noticed that there are some lints in this latest version. You may want to fix those.

[jedel1043]

Rebased

[HalidOdat]

Looks good to me! :)