* Avoid mangling unused parts of pkgs.haskell in `fromHOL`.
* Create 'defComp', specifying the default compiler, customisable
through ./comp.nix.
* Include `modernise` module as a workaround for NIX_GHC -> XMONAD_GHC.
.gitignore:
* Include nix 'result' symlink.
As advised in [1], provide a way to set the path for the xmessage and
ghc binaries via XMONAD_XMESSAGE and XMONAD_GHC environment variables.
[1]: 36d5761b3e