Custom JavaScript string rust type

[HalidOdat]

The current JavaScript string rust (RcString which holds a Rc<str>) is 16 bytes this is because it stores the pointer to the data and length on the stack. And this prevents #1373 from being implemented. So we need to write a custom string type which is referenced counted like Rc<str> and holds the length on the heap.

I should almost be done with this, and I'll create a PR soon :)