Implement generalised list comprehensions

Implement generalised list comprehensions

  This patch implements generalised list comprehensions, as described in
  the paper "Comprehensive comprehensions" (Peyton Jones & Wadler, Haskell
  Workshop 2007).  If you don't use the new comprehensions, nothing
  should change.

  The syntax is not exactly as in the paper; see the user manual entry
  for details.

  You need an accompanying patch to the base library for this stuff
  to work.

  The patch is the work of Max Bolingbroke [batterseapower@hotmail.com],
  with some advice from Simon PJ.

  The related GHC Wiki page is
    http://hackage.haskell.org/trac/ghc/wiki/SQLLikeComprehensions