Improve kind generalisation, error messages

This patch does two things:

* It refactors GHC.Tc.Errors a bit.  In debugging Quick Look I was
  forced to look in detail at error messages, and ended up doing a bit
  of refactoring, esp in mkTyVarEqErr'.  It's still quite a mess, but
  a bit better, I think.

* It makes a significant improvement to the kind checking of type and
  class declarations. Specifically, we now ensure that if kind
  checking fails with an unsolved constraint, all the skolems are in
  scope.  That wasn't the case before, which led to some obscure error
  messages; and occasional failures with "no skolem info" (eg #16245).

Both of these, and the main Quick Look patch itself, affect a /lot/ of
error messages, as you can see from the number of files changed.  I've
checked them all; I think they are as good or better than before.

Smaller things

* I documented the various instances of VarBndr better.
  See Note [The VarBndr tyep and its uses] in GHC.Types.Var

* Renamed GHC.Tc.Solver.simpl_top to simplifyTopWanteds

* A bit of refactoring in bindExplicitTKTele, to avoid the
  footwork with Either.  Simpler now.

* Move promoteTyVar from GHC.Tc.Solver to GHC.Tc.Utils.TcMType

Fixes #16245 (comment 211369), memorialised as
  typecheck/polykinds/T16245a
Also fixes the three bugs in #18640

4 files changed, 5 insertions, 7 deletions

diff --git a/testsuite/tests/typecheck/should_run/T9497a-run.stderr b/testsuite/tests/typecheck/should_run/T9497a-run.stderr
index 6461d98b10..7f05844778 100644
--- a/testsuite/tests/typecheck/should_run/T9497a-run.stderr
+++ b/testsuite/tests/typecheck/should_run/T9497a-run.stderr
@@ -1,8 +1,7 @@
 T9497a-run: T9497a-run.hs:2:8: error:
     • Found hole: _main :: IO ()
       Or perhaps ‘_main’ is mis-spelled, or not in scope
-    • In the expression: _main
-      In an equation for ‘main’: main = _main
+    • In an equation for ‘main’: main = _main
     • Relevant bindings include
         main :: IO () (bound at T9497a-run.hs:2:1)
       Valid hole fits include
diff --git a/testsuite/tests/typecheck/should_run/T9497b-run.stderr b/testsuite/tests/typecheck/should_run/T9497b-run.stderr
index a73b71981c..e6155ddae5 100644
--- a/testsuite/tests/typecheck/should_run/T9497b-run.stderr
+++ b/testsuite/tests/typecheck/should_run/T9497b-run.stderr
@@ -1,8 +1,7 @@
 T9497b-run: T9497b-run.hs:2:8: error:
     • Found hole: _main :: IO ()
       Or perhaps ‘_main’ is mis-spelled, or not in scope
-    • In the expression: _main
-      In an equation for ‘main’: main = _main
+    • In an equation for ‘main’: main = _main
     • Relevant bindings include
         main :: IO () (bound at T9497b-run.hs:2:1)
       Valid hole fits include
diff --git a/testsuite/tests/typecheck/should_run/T9497c-run.stderr b/testsuite/tests/typecheck/should_run/T9497c-run.stderr
index 8666900b4e..6ce2781031 100644
--- a/testsuite/tests/typecheck/should_run/T9497c-run.stderr
+++ b/testsuite/tests/typecheck/should_run/T9497c-run.stderr
@@ -1,8 +1,7 @@
 T9497c-run: T9497c-run.hs:2:8: error:
     • Found hole: _main :: IO ()
       Or perhaps ‘_main’ is mis-spelled, or not in scope
-    • In the expression: _main
-      In an equation for ‘main’: main = _main
+    • In an equation for ‘main’: main = _main
     • Relevant bindings include
         main :: IO () (bound at T9497c-run.hs:2:1)
       Valid hole fits include
diff --git a/testsuite/tests/typecheck/should_run/tcrun042.hs b/testsuite/tests/typecheck/should_run/tcrun042.hs
index 30c67601ed..82add262b1 100644
--- a/testsuite/tests/typecheck/should_run/tcrun042.hs
+++ b/testsuite/tests/typecheck/should_run/tcrun042.hs
@@ -22,4 +22,5 @@ e = (,"Hello" ++ "World",)
 dropFunction :: (a, String, forall c. c -> c -> c) -> (a, String, Int)
 dropFunction (x, y, z) = (x, y, z 10 20)
 
-main = print (dropFunction $ e "Meh" (flip const), dropFunction $ e 10 const)
+main = print ( dropFunction (e "Meh" (flip const))
+             , dropFunction (e 10 const))