Promoted classes without Proxy

[int-index]

closes #149

In the light of https://ghc.haskell.org/trac/ghc/ticket/12928, I decided to proceed as follows:

• if a class type variable has no explicit kind, we make no effort to guess it and default to *. This is OK because before TypeInType we were limited by KProxy anyway.
• if a class type variable has an explicit kind, it is preserved.

This way, we always get CUSKs where we need them.

I also modified promoteType slightly, so this PR closes #145, because now this code compiles:

{-# LANGUAGE TypeInType, ScopedTypeVariables, TypeFamilies, GADTs, TemplateHaskell #-}

import Data.Kind
import Data.Singletons.Promote

$(promote [d|
  class Column (f :: Type -> Type) where
    col :: f a -> a -> Bool
  |])

[int-index]

Just to make it clear, this version does not remove KindInference constructors.

[goldfirere]

This looks fantastic. Thanks so much. Do check out my comment in promoteType, though, as I think there's a flat-out error there.

[goldfirere]

This seems wrong. It will treat (x :: k) y as (x y :: k), I think.

[int-index]

To be honest, I didn't entirely understand what I was doing here. I'll add a test.

[goldfirere]

The one use of this function will be able to removed once goldfirere/th-desugar#47 is merged, I think. I don't love the one-off use of the Proxy names here, but it should be temporary enough.

[int-index]

You are right, I intended this function to be temporary, which is why it's in an inappropriate module and uses Proxy names this way. Looking forward to getting rid of it.

[goldfirere]

Yay! Thanks!