Const generics

[varkor]

With work on const generics underway in rustc, we'll want to make sure Chalk also supports them.

I've started looking into this here, starting off by extending ParameterKind and filling in things from there. I'll give an update when progress is a little further along.

[AzureMarker]

#393 has been merged, can this be closed?

[nikomatsakis]

I guess so -- I don't think we've completed the story here, but at the same time I think we need a fresh tracking issue with more detail, and that issue may even want to live in the wg-traits repository.