Convert RsAsyncFunction from `fn` to `Fn` (function pointer to trait object)

[eirikb]

This helps passing or sharing state to function of things that uses this, e.g., register_async_function. Have to be Box or else there will be Size issues.

@rscarson hopefully this doesn't break any tests - or anything else 😅

[eirikb]

I see you fixed this, thanks :) Closing