Try to speed up evaluation

[Chris Warburton]

Evaluation of 'import <nixpkgs> { config = import "this repo"; }' is
very slow. Running Nix commands with the '-v' flag shows that we're
loading a whole bunch of stuff from many different versions of nixpkgs.

Whilst we want multiple versions to be available, we should try to avoid
actually using them for our day-to-day config. We can do this in two
ways:

 - For things which just-so-happen to use old versions, we can try
   bumping them to use 'super' or 'self' instead. Or, you know, get rid
   of pinned versions if there's no need to pin them.

 - For things which are specifically broken or missing on newer nixpkgs
   versions, we could try importing just the relevant file, rather than
   the whole config.

An example of the latter might be:

    { nixpkgs1603 }:

    # This was removed in nixpkgs 16.09
    nixpkgs1603.fooPackages.bar

We might try doing this instead:

    { fooPackages, repo1603 }:

    # This was removed in nixpkgs 16.09
    fooPackages.callPackage "${repo1603}/pkgs/foo/bar.nix" {}

This way, we're being much more targeted. Dependencies might be an
issue, so this isn't guaranteed to work, but I think it's worth a shot.