Refactor default methods (Trac #11105)

This patch does some signficant refactoring to the treatment
of default methods in class declarations, and more generally
to the type checking of type/class decls.

Highlights:

* When the class has a generic-default method, such as
     class C a where
       op :: a -> a -> Bool
       default op :: Ord a => a -> a -> a
  the ClassOpItem records the type of the generic-default,
  in this case the type (Ord a => a -> a -> a)

* I killed off Class.DefMeth in favour of the very-similar
  BasicTypes.DefMethSpec.  However it turned out to be better
  to use a Maybe, thus
      Maybe (DefMethSpec Type)
  with Nothing meaning "no default method".

* In TcTyClsDecls.tcTyClGroup, we used to accumulate a [TyThing],
  but I found a way to make it much simpler, accumulating only
  a [TyCon].  Much less wrapping and unwrapping.

* On the way I also fixed Trac #10896 in a better way. Instead
  of killing off all ambiguity checks whenever there are any type
  errors (the fix in commit 8e8b9ed), I instead recover in
  TcTyClsDecls.checkValidTyCl.

There was a lot of associated simplification all round

1 files changed, 0 insertions, 11 deletions

diff --git a/compiler/utils/Binary.hs b/compiler/utils/Binary.hs
index 5083804d6f..ab5b772eec 100644
--- a/compiler/utils/Binary.hs
+++ b/compiler/utils/Binary.hs
@@ -807,17 +807,6 @@ instance Binary InlineSpec where
                   2 -> return Inlinable
                   _ -> return NoInline
 
-instance Binary DefMethSpec where
-    put_ bh NoDM      = putByte bh 0
-    put_ bh VanillaDM = putByte bh 1
-    put_ bh GenericDM = putByte bh 2
-    get bh = do
-            h <- getByte bh
-            case h of
-              0 -> return NoDM
-              1 -> return VanillaDM
-              _ -> return GenericDM
-
 instance Binary RecFlag where
     put_ bh Recursive = do
             putByte bh 0