Remove "Data t =>" qualifications from transformation types.

This is solely because GHC 6.6 doesn't like them (it complains about the type
variable already being in scope -- which it is, but there's nothing I can do
about that!). This doesn't lose any safety; if you try to write a
transformation for something that's not Data you'll find out when you try to
pass it to one of the application functions.

pass/Traversal.hs

@@ -33,17 +33,16 @@ import Pass
 -- | A transformation for a single 'Data' type with explicit descent.
 -- The first argument passed is a function that can be called to explicitly
 -- descend into a generic value.
-type ExplicitTrans t = Data t =>
+type ExplicitTrans t = (forall s. Data s => s -> PassM s) -> t -> PassM t
-                       (forall s. Data s => s -> PassM s) -> t -> PassM t
 
 -- | A transformation for a single 'Data' type with implicit descent.
 -- This can be applied recursively throughout a data structure.
-type Transform t = Data t => t -> PassM t
+type Transform t = t -> PassM t
 
 -- | A check for a single 'Data' type with implicit descent.
 -- This is like 'Transform', but it doesn't change the value; it may fail or
 -- modify the state, though.
-type Check t = Data t => t -> PassM ()
+type Check t = t -> PassM ()
 
 -- | Make an 'ExplicitTrans' that applies a 'Transform', recursing depth-first.
 transformToExplicitDepth :: Data t => Transform t -> ExplicitTrans t