Add CachedRef borrowing

[tynberry]

When using CachedRef one had to pass a closure or a function that takes the mutable reference to the component and returns something. This limits ergonomics as one has to either extract fields from the component, possibly requiring to clone them which might be expensive or move their entire code into the closure.

This adds borrow_mut and try_borrow_mut methods which return a mutable reference to the component (returning an immutable reference is useless as it would not make the method &self instead of &mut self).

If there are any unsoudness issues with this approach, notice me, this is my first time contributing to Flecs.