Mutable instance captures

[Tamschi]

If a field should be modified, that's usually written as |value = RefCell::<i32>::new(initial)|; or similar.
(Technically using an atomic would be better in this case, but that's more verbose elsewhere.)

It should be possible to declare mutable instance fields more quickly, for example like so |mut value: i32 = {initial}|.

I haven't yet looked into the implications of adding a feature like this. Components with mutable fields would probably have to contain a RefCell instance, or alternatively I could confine mutable components to the current thread (which wouldn't be a big loss, since Asteracea targets primarily the single-threaded web platform in terms of rendering and event handlers shouldn't have to be Send or Sync either).

[Tamschi]

This should use Cell, not RefCell.