This includes updates to elm -compiler, -make, -package and -reactor.
-repl's test suite currently breaks.
The build for elm-reactor could be more concise, but it's clean. The
final build step for elm-reactor ultimately relies on elm-package
downloading some libraries, so anything radically better would probably
need nix to step in and somehow emulate elm-package. That would take
more knowledge about elm & nix than I have...
