Extract class macro into a library

[dherman]

With a smaller primitive built into Neon, can we move the class macro out into a library? It's not fully baked and it would be good to avoid permanently polluting the user-facing Neon design with it.

Depends on #561.

[dherman]

We're going to just document the idea of this in the transition guide, but it doesn't seem worth the effort to build a full compatibility layer, at least not unless a user raises the issue.