Refactor Context usage for performance

[jamesplease]

Resolves #21

The problem

The old implementation propagated changes through Context. This was problematic in situations where there were a lot of focused nodes: any change to the focus tree would rerender every subscribed component.

I was observing noticeable drops in performance due to this problem.

This solution

In this solution, Context never causes a rerender, because the Context values are always static.

Instead, localized state causes a render. This state is kept in sync using focusTree.subscribe(). To ensure that the fewest number of things render at a time, a comparison check is utilized in the subscribe callback to see if state should be synchronized.

Additional work

It would probably be worthwhile to refactor the Focusable in a more substantial way. Perhaps I'll do that when I work on #25

Alternative solutions

Context selectors is an alternative solution, but as of the time of writing they are only an RFC.