Come up with a "roadmap" to the Typelevel ecosystem #597

(This is a prerequisite to many other tasks; it isn't clear what its own final artifact is beyond a shared internal document.)

We have many, many, many projects. If we're going to talk about them publicly in a coherent way, we need to organize our thoughts about how they all work together. This involves several aspects:

• Categorizing them, so that we can talk about them coherently.
• Getting clear what each is good for. What use cases is each one strongest for?
• Seriously talking about the Typelevel "platform". For different sorts of application, what libraries should you be considering?

The last point is probably most crucial. We need to understand, very clearly, how the pieces work together, so that we can talk about this intelligently and make recommendations to folks who want to build serious software with TL libraries as the base.