Added ability to swap stores
Removed single-child limitation

Added invariant warnings for storeKey and withRef
Added valid element check using react-is
Refactored child selector creation for reusability
Added prop types for components
Added forwardRef and consumer as prop capability
Added a tiny memoized function for wrapper props handling
Removed semicolons

Replaced multiple levels of class wrappers with a simple hook that
reads the store state from context, initializes and uses the
provided selector, and memoizes the wrapped component as needed.

Updated Provider and connect to accept an entire context object
as a prop, because `useContext` only works with a context object
and not just the consumer.

Also allows passing a new store instance to Provider, because we
can easily reset the subscription aspect now.

Updated tests.