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Regression testing and user feedback for GHC 7.0 taught
us a lot.  This patch fixes numerous small bugs, and some
major ones (eg Trac #4484, #4492), and improves type
error messages.

The main changes are:

* Entirely remove the "skolem equivalance class" stuff;
  a very useful simplification

* Instead, when flattening "wanted" constraints we generate
  unification variables (not flatten-skolems) for the
  flattened type function application

* We then need a fixup pass at the end, TcSimplify.solveCTyFunEqs,
  which resolves any residual equalities of form
      F xi ~ alpha

* When we come across a definite failure (e.g. Int ~ [a]),
  we now defer reporting the error until the end, in case we
  learn more about 'a'.  That is particularly important for
  occurs-check errors.  These are called "frozen" type errors.

* Other improvements in error message generation.

* Better tracing messages

In particular, instead of
   Cannot match 'a' with 'b'
we get
   Could not deduce (a~b) from context (F a ~ b)
or whatever

A bit yukky; see Note [Runtime skolems] in TcErrors.
But it works, and the debugger just is yukky in places.

Major pass through type checker:(1) prioritizing equalities, (2) improved Derived mechanism, (3) bugfixes

The error cascade caused puzzling errors in T4093b, and
suppressing some seems like a good plan.  Very few test
outputs change.

and remove the temporary DOpt class workaround.

Improves kind error messages in paticular

This major patch implements the new OutsideIn constraint solving
algorithm in the typecheker, following our JFP paper "Modular type
inference with local assumptions".  

Done with major help from Dimitrios Vytiniotis and Brent Yorgey.