haskell: add support for all of Hackage

The function "callHackage <name> <version>" generates build instructions for
the requested library version on-the-fly. All of Hackage is available. Note:
this code is brand-new, experimental, and it might change in the future. Don't
base production code on this feature yet.

Example usage:

  $ nix-shell -p 'haskellPackages.callHackage "cpphs" "1.19.3" {}' --run "cpphs --version"
  cpphs 1.19.3

  $ nix-shell -p 'haskellPackages.ghcWithPackages (self: [(self.callHackage "hsdns" "1.6.1" {})])' --run "ghc-pkg list hsdns"
  /nix/store/p6r81k2vb2pzy4wcvri6z9m492i0hg63-ghc-8.0.1/lib/ghc-8.0.1/package.conf.d
      hsdns-1.6.1