Register and create closure functions

[HalidOdat]

Currently we can only create function that can be coerced into a NativeFunction, this does not allow us to capture variables from a closure. And changing the function signature of NativeFunction to Box<dyn Fn> would be bad for performance reasons, since for every function now we have to allocate. Instead we would have another variant in Function enum Closure that would contain a Box<dyn Fn>

So I propose the following, we add separate functions for closures function creation.

let some_variable = ...
context.register_global_closure("name", move |_, _, _| {
	Ok(some_variable)
});

We would also add a way to create a closure with the FunctionBuilder