[Nix-dev] Simplified 'ghcWithPackages' wrapper

[Peter Simons]

Hi guys,

I have committed two changes to stdenv-updates that simply the
implementation of the 'ghcWithPackages' functions quite a bit:

  90fc42c2f2bef844f5689b5939bcf760a23e4a63
  d09269a22eed2e844b046c31899887aa6c1fb42d

I'm moderately confident that those changes don't break anything
(i.e. I believe that the traditional ghc-wrapper script still works
okay), but I'd appreciate some feedback if any of you find issues.

Take care,
Peter