Ryan Scott authored Feb 09, 2019 and Marge Bot committed Feb 12, 2019

This bug fixes three problems related to `Proxy#`/`proxy#`:

1. Reifying it with TH claims that the `Proxy#` type constructor has
   two arguments, but that ought to be one for consistency with
   TH's treatment for other primitive type constructors like `(->)`.
   This was fixed by just returning the number of
   `tyConVisibleTyVars` instead of using `tyConArity` (which includes
   invisible arguments).
2. The role of `Proxy#`'s visible argument was hard-coded as nominal.
   Easily fixed by changing it to phantom.
3. The visibility of `proxy#`'s kind argument was specified, which
   is different from the `Proxy` constructor (which treats it as
   inferred). Some minor refactoring in `proxyHashId` fixed ths up.

   Along the way, I had to introduce a `mkSpecForAllTy` function, so
   I did some related Haddock cleanup in `Type`, where that function
   lives.