Give an error message for INLINE/SPECIALISE for missing default method

Basically, if the user has written:

  class Foo a where
    bar :: a -> a
    {-# INLINE bar #-}

Then we should error out because there is no default method corresponding
to the `bar' INLINE pragma. This patch achieves this by splitting the
signatures for a class declaration apart into two sets: one that applies
to the defaults (INLINE, SPECIALISE), and one which defines the class itself
(fixity, type signatures). The two sets are then renamed in different contexts.