Include selectors for working with entity reducer

[devinivy]

It would be great if selectors existed to get a specific record by id, check if an index is loading or initialized, and get shallow or deep results from an index.

[wswoodruff]

After speaking with @devinivy, this task's solution should be much simpler than his own project denormie — and should act like a utility like we've used for internals.loading in the past.