Resolve the Col var story

[nfrisby]

I defined the .& and .= families separately so that we could have the rather natural syntax that we have.

But doing it this way opens the possibility of type variables inhabiting the Col kind, when I really don't think those will ever be interesting. For now, I've asked the user (in Haddock) to avoid such Col vars.

But the plugin code still has some old attempts at worrying about this. I think it's basically dead code. (E.G. Some of it implements injectivity of the .= family; but I think the type family dependencies now make that redundant.)

This task is to decide on how best to handle Col vars and to then prune/simplify that code in the plugin.