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	Lift the class-method type restriction
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Haskell 98 prohibits class method types to mention constraints on the
class type variable, thus:

  class Seq s a where
    fromList :: [a] -> s a
    elem     :: Eq a => a -> s a -> Bool

The type of 'elem' is illegal in Haskell 98, because it contains the
constraint 'Eq a', which constrains only the class type variable (in
this case 'a').

This commit lifts the restriction.  The way we do that is to do a full
context reduction (tcSimplifyCheck) step for each method separately in
TcClassDcl.tcMethodBind, rather than doing a single context reduction
for the whole group of method bindings.

As a result, I had to reorganise the code a bit, and tidy up.