Improve improvement in the constraint solver

This regrettably-big patch substantially improves the way in which
"improvement" happens in the constraint solver.  It was triggered by
trying to crack Trac #10009, but it turned out to solve #10340 as
well.

The big picture, with several of the trickiest examples, is described
in Note [The improvement story] in TcInteract.

The major change is this:

* After solving we explicitly try "improvement", by
     - making the unsolved Wanteds into Deriveds
     - allowing Deriveds to rewrite Deriveds
  This more aggressive rewriting "unlocks" some extra
  guess-free unifications.

* The main loop is in TcInteract.solveSimpleWanteds, but I also ended
  up refactoring TcSimplify.simpl_loop, and its surrounding code.

  Notably, any insolubles from the Givens are pulled out
  and treated separately, rather than staying in the inert set
  during the solveSimpleWanteds loop.

There are a lot of follow-on changes

* Do not emit generate Derived improvements from Wanteds.
  This saves work in the common case where they aren't needed.

* For improvement we should really do type-class reduction on Derived
  constraints in doTopReactDict.  That entailed changing the GenInst
  constructor a bit; a local and minor change

* Some annoying faffing about with dropping derived constraints;
  see dropDerivedWC, dropDerivedSimples, dropDerivedInsols,
  and their Notes.

* Some substantial refactoring in TcErrors.reportWanteds.
  This work wasn't strictly forced, but I got sucked into it.
  All the changes are in TcErrors.

* Use TcS.unifyTyVar consistently, rather than setWantedTyBind,
  so that unifications are properly tracked.

* Refactoring around solveWantedsTcM, solveWantedsAndDrop.
  They previously guaranteed a zonked result, but it's more
  straightforward for clients to zonk.

1 files changed, 2 insertions, 2 deletions

diff --git a/testsuite/tests/typecheck/should_fail/mc25.stderr b/testsuite/tests/typecheck/should_fail/mc25.stderr
index 83fe1301d6..ec88439e89 100644
--- a/testsuite/tests/typecheck/should_fail/mc25.stderr
+++ b/testsuite/tests/typecheck/should_fail/mc25.stderr
@@ -1,5 +1,5 @@
 
-mc25.hs:9:10:
+mc25.hs:9:10: error:
     No instance for (Functor t1) arising from a use of ‘fmap’
     Possible fix:
       add (Functor t1) to the context of
@@ -9,7 +9,7 @@ mc25.hs:9:10:
     In a stmt of a monad comprehension: then group by x using take
     In the expression: [x | x <- [1 .. 10], then group by x using take]
 
-mc25.hs:9:46:
+mc25.hs:9:46: error:
     Couldn't match type ‘a -> t’ with ‘Int’
     Expected type: (a -> t) -> [a] -> [t1 a]
       Actual type: Int -> [t1 a] -> [t1 a]