Add ways to build hadrian using nix (#473)

* Add ways to build hadrian using nix

This adds two new files to the hadrian directory

    shell.nix sets up the build envrionment you need to build ghc
    build-nix is a simple wrapper which invokes hadrian in the correct environment

Note: this patch was authored by @mpickering, however it ended up on phabricator due to the subtree as https://phabricator.haskell.org/D4207.

* Comments and renamed build-nix