Substantial improvements to coercion optimisation

The main purpose of this patch is to add a bunch of new rules
to the coercion optimiser.  They are documented in the (revised)
Appendix of the System FC paper.  

Some code has moved about:

- OptCoercion is now a separate module, mainly because it
  now uses tcMatchTy, which is defined in Unify, so OptCoercion
  must live higehr up in the hierarchy

- Functions that manipulate Kinds has moved from 
  Type.lhs to Coercion.lhs.  Reason: the function typeKind
  now needs to call coercionKind.  And in any case, a Kind is
  a flavour of Type, so it builds on top of Type; indeed Coercions
  and Kinds are both flavours of Type.

  This change required fiddling with a number of imports, hence
  the one-line changes to otherwise-unrelated modules

- The representation of CoTyCons in TyCon has changed.   Instead of
  an extensional representation (a kind checker) there is now an
  intensional representation (namely TyCon.CoTyConDesc).  This was
  needed for one of the new coercion optimisations.

2 files changed, 4 insertions, 2 deletions

diff --git a/compiler/parser/Parser.y.pp b/compiler/parser/Parser.y.pp
index 4cd637fb34..212a79ce36 100644
--- a/compiler/parser/Parser.y.pp
+++ b/compiler/parser/Parser.y.pp
@@ -45,7 +45,8 @@ import SrcLoc		( Located(..), unLoc, getLoc, noLoc, combineSrcSpans,
 			  mkSrcLoc, mkSrcSpan )
 import Module
 import StaticFlags	( opt_SccProfilingOn, opt_Hpc )
-import Type		( Kind, mkArrowKind, liftedTypeKind, unliftedTypeKind )
+import Type		( Kind, liftedTypeKind, unliftedTypeKind )
+import Coercion		( mkArrowKind )
 import Class		( FunDep )
 import BasicTypes	( Boxity(..), Fixity(..), FixityDirection(..), IPName(..),
 			  Activation(..), RuleMatchInfo(..), defaultInlinePragma )
diff --git a/compiler/parser/ParserCore.y b/compiler/parser/ParserCore.y
index 6839fa2d45..f43e225041 100644
--- a/compiler/parser/ParserCore.y
+++ b/compiler/parser/ParserCore.y
@@ -16,8 +16,9 @@ import RdrName
 import OccName
 import Type ( Kind,
               liftedTypeKindTyCon, openTypeKindTyCon, unliftedTypeKindTyCon,
-              argTypeKindTyCon, ubxTupleKindTyCon, mkArrowKind, mkTyConApp
+              argTypeKindTyCon, ubxTupleKindTyCon, mkTyConApp
             )
+import Coercion( mkArrowKind )
 import Name( Name, nameOccName, nameModule, mkExternalName )
 import Module
 import ParserCoreUtils