Consistent Widget::diff logic

[hecrj]

This PR changes the Widget trait to call diff logic consistently after every view with a mutable self.

Effectively, this makes the diff method a lot more powerful; since it can now be used to run initialization logic in a consistent way.

For instance, sizing inheritance has been moved from widget tree construction to the diff method. This allows dynamic widgets like component (and the upcoming transition in #3344) to properly report their final size, since they now have a chance to access persistent state.

However, given that now sizing inheritance is done after widget construction, we need a way to differentiate between Shrink and the default "inherited sizing" strategy of containers. This prompted the introduction of a new Length variant called Fit.

Using Fit should be a no-op in most cases, since it's the default sizing strategy of most widgets. However, Fit will become quite useful soon.

Furthermore, since diff is now called after every view, the children method in Widget becomes redundant. We can simply initialize Tree::children as an empty Vec and let diff do the rest–removing the need to keep both children and diff logic synchronized.

Finally, since sizing inheritance can be properly done during diff, we can also get rid of the size_hint hack.

Lots of things clicking into place here!