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, 3 insertions, 11 deletions

diff --git a/testsuite/tests/module/mod71.stderr b/testsuite/tests/module/mod71.stderr
index 12962aa473..53d697e133 100644
--- a/testsuite/tests/module/mod71.stderr
+++ b/testsuite/tests/module/mod71.stderr
@@ -1,20 +1,12 @@
 
-mod71.hs:4:9:
+mod71.hs:4:9: error:
     Found hole ‘_’ with type: t1
     Where: ‘t1’ is a rigid type variable bound by
-                the inferred type of f :: (t1 -> a -> t) -> t at mod71.hs:4:1
+                the inferred type of f :: Num a => (t1 -> a -> t) -> t
+                at mod71.hs:4:1
     Relevant bindings include
       x :: t1 -> a -> t (bound at mod71.hs:4:3)
       f :: (t1 -> a -> t) -> t (bound at mod71.hs:4:1)
     In the first argument of ‘x’, namely ‘_’
     In the expression: x _ 1
     In an equation for ‘f’: f x = x _ 1
-
-mod71.hs:4:11:
-    No instance for (Num a) arising from the literal ‘1’
-    Possible fix:
-      add (Num a) to the context of
-        the inferred type of f :: (t1 -> a -> t) -> t
-    In the second argument of ‘x’, namely ‘1’
-    In the expression: x _ 1
-    In an equation for ‘f’: f x = x _ 1