Weaken constraints on Data.Complex functions

The RealFloat data type context was dropped from Data.Complex some time
ago (see ea280135dd888ac41d8804a9e37e358180cf13ac). However, the rest of
the API in that module was left as-is even though many of the accessors
in that module would work with much more general types now.

This change simplifies the signatures of the functions above, which in
the current API all unnecessarily take a RealFloat constraint that they
don't use (and which may cause to pass around superflous type-class
dictionaries):

    realPart  :: Complex a -> a
    imagPart  :: Complex a -> a
    conjugate :: Num a => Complex a -> Complex a
    mkPolar   :: Floating a => a -> a -> Complex a
    cis       :: Floating a => a -> Complex a

This was originally proposed by Edward Kmett in

  http://www.haskell.org/pipermail/libraries/2014-March/022358.html

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>