Refactor some cruft in TcDerivInfer.inferConstraints

The latest installment in my quest to clean up the code in
`TcDeriv*`. This time, my sights are set on
`TcDerivInfer.inferConstraints`, which infers the context for derived
instances. This function is a wee bit awkward at the moment:

* It's not terribly obvious from a quick glance, but
  `inferConstraints` is only ever invoked when using the `stock` or
  `anyclass` deriving strategies, as the code for inferring the
  context for `newtype`- or `via`-derived instances is located
  separately in `mk_coerce_based_eqn`. But there's no good reason
  for things to be this way, so I moved this code from
  `mk_coerce_based_eqn` to `inferConstraints` so that everything
  related to inferring instance contexts is located in one place.
* In this process, I discovered that the Haddocks for the auxiliary
  function `inferConstraintsDataConArgs` are completely wrong. It
  claims that it handles both `stock` and `newtype` deriving, but
  this is completely wrong, as discussed above—it only handles
  `stock`. To rectify this, I renamed this function to
  `inferConstraintsStock` to reflect its actual purpose and created
  a new `inferConstraintsCoerceBased` function to specifically
  handle `newtype` (and `via`) deriving.

Doing this revealed some opportunities for further simplification:

* Removing the context-inference–related code from
  `mk_coerce_based_eqn` made me realize that the overall structure
  of the function is basically identical to `mk_originative_eqn`.
  In fact, I was easily able to combine the two functions into a
  single `mk_eqn_from_mechanism` function.

  As part of this merger, I now invoke
  `atf_coerce_based_error_checks` from `doDerivInstErrorChecks1`.
* I discovered that GHC defined this function:

  ```hs
  typeToTypeKind = liftedTypeKind `mkVisFunTy` liftedTypeKind
  ```

  No fewer than four times in different modules. I consolidated all
  of these definitions in a single location in `TysWiredIn`.

1 files changed, 4 insertions, 2 deletions

diff --git a/compiler/prelude/TysWiredIn.hs b/compiler/prelude/TysWiredIn.hs
index be2c7eb735..e42009fa61 100644
--- a/compiler/prelude/TysWiredIn.hs
+++ b/compiler/prelude/TysWiredIn.hs
@@ -92,7 +92,8 @@ module TysWiredIn (
 
         -- * Kinds
         typeNatKindCon, typeNatKind, typeSymbolKindCon, typeSymbolKind,
-        isLiftedTypeKindTyConName, liftedTypeKind, constraintKind,
+        isLiftedTypeKindTyConName, liftedTypeKind,
+        typeToTypeKind, constraintKind,
         liftedTypeKindTyCon, constraintKindTyCon,  constraintKindTyConName,
         liftedTypeKindTyConName,
 
@@ -612,8 +613,9 @@ typeSymbolKind = mkTyConTy typeSymbolKindCon
 constraintKindTyCon :: TyCon
 constraintKindTyCon = pcTyCon constraintKindTyConName Nothing [] []
 
-liftedTypeKind, constraintKind :: Kind
+liftedTypeKind, typeToTypeKind, constraintKind :: Kind
 liftedTypeKind   = tYPE liftedRepTy
+typeToTypeKind   = liftedTypeKind `mkVisFunTy` liftedTypeKind
 constraintKind   = mkTyConApp constraintKindTyCon []
 
 {-