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, 2 insertions, 2 deletions

diff --git a/testsuite/tests/typecheck/should_fail/tcfail143.stderr b/testsuite/tests/typecheck/should_fail/tcfail143.stderr
index 7c26762a66..14c73d98da 100644
--- a/testsuite/tests/typecheck/should_fail/tcfail143.stderr
+++ b/testsuite/tests/typecheck/should_fail/tcfail143.stderr
@@ -1,8 +1,8 @@
 
 tcfail143.hs:29:9: error:
-    Couldn't match type ‘Z’ with ‘S Z’
+    Couldn't match type ‘S Z’ with ‘Z’
     arising from a functional dependency between:
       constraint ‘MinMax (S Z) Z Z Z’ arising from a use of ‘extend’
-      instance ‘MinMax Z b Z b’ at tcfail143.hs:12:10-23
+      instance ‘MinMax a Z Z a’ at tcfail143.hs:11:10-23
     In the expression: n1 `extend` n0
     In an equation for ‘t2’: t2 = n1 `extend` n0