patch applied (packages/mtl): make it build with versions of base before and after 4.3

[Ross Paterson]

Fri Sep  3 04:33:58 PDT 2010  Ross Paterson <ross at soi.city.ac.uk>
  * make it build with versions of base before and after 4.3
  
  The Monad and MonadFix instances for Either moved to base-4.3, and
  lost their Error constraints.  Test the version of base to either
  supply the instances here or use the ones from base.

    M ./Control/Monad/Error.hs +20
    M ./mtl.cabal -1 +1

View patch online:
http://darcs.haskell.org/cgi-bin/darcsweb.cgi?r=packages/mtl;a=darcs_commitdiff;h=20100903113358-b47d3-b51f5c2cdefe8e7d9c14e5633b782799eeff09ed.gz