Utilize multiple contexts for performance

[jamesplease]

Consider the following:

• Subtree A is focused
• Subtree A root has a hook checking if it is focused
• Subtree B is changing dynamically based on user input, and many, many focusable elements are being created/destroyed

In this situation, Subtree B is changing the global context (because the focus tree nodes are changing), causing Subtree A to render unnecessarily.

The focus hierarchy state should be separate from the node state. That way, consumers of what's focused won't be affected by changes to the rest of the focus tree state.

[jamesplease]

This change may still cause A to rerender because A subscribes to get its own node within a Focusable component.

[jamesplease]

A more appropriate solution is described here: reactjs/rfcs#119

However, since that is just an RFC, I'll likely move to:

1. passing down the entire tree through Context, which will never re-render
2. storing Node information in state within the custom hooks/Focusable component
3. syncing that state using a selector approach

Each hook/Focusable will subscribe to the Focus Tree rather than Context. In this way, re-renders will be minimized considerably.

If the selector RFC ever lands, I can refactor back to using Context in a less roundabout way.