Another major improvement of "improvement"

I wasn't very happy with my fix to Trac #10009. This is much better.

The main idea is that the inert set now contains a "model", which
embodies *all* the (nominal) equalities that we know about, with
a view to exposing unifications.  This requires a lot fewer iterations
of the solver than before.

There are extensive comments in
 TcSMonad:  Note [inert_model: the inert model]
            Note [Adding an inert canonical constraint the InertCans]

The big changes are

  * New inert_model field in InertCans

  * Functions addInertEq, addInertCan deal with adding a
    constraint, maintaining the model

  * A nice improvement is that unification variables can
    unify with fmvs, so that from, say   alpha ~ fmv
    we get              alpha := fmv
    See Note [Orientation of equalities with fmvs] in TcFlatten
    It's still not perfect, as the Note explains

New flag -fconstraint-solver-iterations=n, allows us to control
the number of constraint solver iterations, and in particular
will flag up when it's more than a small number.

Performance is generally slightly better:
T5837 is a lot better for some reason.

1 files changed, 22 insertions, 11 deletions

diff --git a/testsuite/tests/indexed-types/should_fail/T2544.stderr b/testsuite/tests/indexed-types/should_fail/T2544.stderr
index 7a1f564c56..0245cb7b6a 100644
--- a/testsuite/tests/indexed-types/should_fail/T2544.stderr
+++ b/testsuite/tests/indexed-types/should_fail/T2544.stderr
@@ -1,11 +1,22 @@
-

-T2544.hs:17:12:

-    Couldn't match type ‘IxMap l’ with ‘IxMap i0’

-    NB: ‘IxMap’ is a type function, and may not be injective

-    The type variable ‘i0’ is ambiguous

-    Expected type: IxMap (l :|: r) [Int]

-      Actual type: BiApp (IxMap i0) (IxMap i1) [Int]

-    Relevant bindings include

-      empty :: IxMap (l :|: r) [Int] (bound at T2544.hs:17:4)

-    In the expression: BiApp empty empty

-    In an equation for ‘empty’: empty = BiApp empty empty

+
+T2544.hs:17:18: error:
+    Couldn't match type ‘IxMap i0’ with ‘IxMap l’
+    NB: ‘IxMap’ is a type function, and may not be injective
+    The type variable ‘i0’ is ambiguous
+    Expected type: IxMap l [Int]
+      Actual type: IxMap i0 [Int]
+    Relevant bindings include
+      empty :: IxMap (l :|: r) [Int] (bound at T2544.hs:17:4)
+    In the first argument of ‘BiApp’, namely ‘empty’
+    In the expression: BiApp empty empty
+
+T2544.hs:17:24: error:
+    Couldn't match type ‘IxMap i1’ with ‘IxMap r’
+    NB: ‘IxMap’ is a type function, and may not be injective
+    The type variable ‘i1’ is ambiguous
+    Expected type: IxMap r [Int]
+      Actual type: IxMap i1 [Int]
+    Relevant bindings include
+      empty :: IxMap (l :|: r) [Int] (bound at T2544.hs:17:4)
+    In the second argument of ‘BiApp’, namely ‘empty’
+    In the expression: BiApp empty empty