Type-safe function binding

[Bromeon]

The statement let result: Result<R, _> = script.call("function", &args); is good enough for simple scripts or plugins with one entry point, but doesn't scale well for plugin APIs (i.e. a set of functions).

Instead of letting the users write type-safe wrappers around call() statements, allow to simply declare the functions and generate the binding from it. Something along this might be nice (still need to check feasibility):

js_api! {
	trait MyApi {
		fn triple(num: i32) -> i32;
		// more functions...
	}
}

fn main() {
	let script: Script = ...;

	// Link the trait to the script, possibly verify functions exist
	let api: Box<dyn MyApi> = script.bind_functions::<dyn MyApi>()?;
	
	// Call directly on the polymorphic trait object
	let x = api.triple(4)?;
}

Some points would need to be clarified:

• generating binding in an easy way
• is there a way to verify a binding before calling functions? (JS doesn't have typed signatures)
• how to handle failing calls (exception, mismatched type)? always return Result?

[Bromeon]

Implemented in latest master and release 0.2.0-rc.0.