Make debugging selectors easy in development mode

[nvie]

One of the easiest things to mess up performance is forgetting to add the shallow argument to your selector function if you're using a .map() or .filter() call in there, or when you want to select multiple subtrees.

To make debugging this easy, we could allow opting-in to a "StrictMode" like setup. When running in StrictMode, only during local development, each selector function would internally execute twice and is expected to be consistent (= both runs should compare equal using the provided isEqual helper). If not, it's tried with a shallow comparison to see if that would lead to the correct result. If so, we can emit a console error which hints at the developer that they may be forgetting the shallow arg.

I implemented a quick and dirty proof of concept for this feature, and I think it's promising. Of course, this needs to be polished before it's ready to ship, but I think people will love it to build performant apps.