WIP: Require GHC 8.6

Don't merge yet.

This checks off all boxes in #324 except the third one (involving
`ShowSing`, which I'll tackle separately). Specifically, this:

* Removes some CPP needed for GHC 8.4.
* Bumps the `th-desugar` submodule.
* Singling `DerivingVia` does nothing (but emits a warning).
* Promoting or singling a quantified constraint results in an error.
* Now that GHC is smarter about kind inference in CUSKs, we can
  remove some superfluous kind signatures on certain class methods
  in `Applicative`, `Monad`, etc., as well as on definitions
  involving `Const`.
* Puts a mention in the `README` of how `NoStarIsType` is needed to
  use `PNum(type (*))`.

cabal.project

@@ -1,3 +1,17 @@
+-- generated by head.hackage.sh
+
+with-compiler: /opt/ghc/8.6.1/bin/ghc
+
 packages: .
           ./th-desugar
           ./util
+
+repository head.hackage
+   url: http://head.hackage.haskell.org/
+   secure: True
+   root-keys: 07c59cb65787dedfaef5bd5f987ceb5f7e5ebf88b904bbd4c5cbdeb2ff71b740
+              2e8555dde16ebd8df076f1a8ef13b8f14c66bad8eafefd7d9e37d0ed711821fb
+              8f79fd2389ab2967354407ec852cbe73f2e8635793ac446d09461ffb99527f6e
+   key-threshold: 3
+
+-- end of generated content