Move store state outside of context and force updates to subscribed components externally

[Jahans3]

This library already borrows heavily from the Redux API that many are familiar with, so in order to get our main hook working as intended (enable partially subscribing to state updates) we may as well borrow some more ideas.

Some key points:

• State will live outside the component hierarchy
• Context will hold a constant reference to the store - the actual context value will be static
• Internally, we will move to a pub/sub architecture
• Subscribers will be forced to re-render from an external subscription - we can enable this by passing a setState to each subscription

The external API should not change and everything should remain simple and quick to setup, meaning store creation should still involve nothing more than passing a reducers prop to our provider.

[Jahans3]

After a long wait this will soon be possible with the introduction of React.useContextSelector