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...
The elm-reactor setup.hs file tries to build an elm file that's not
distributed in the cabal archive. The archive contains the
already-compiled js file so the step is unnecessary and I'm patching
it out.
Note that elm has a different package split: The old Elm 0.13 package
is now elm-compiler, elm-make and elm-package.
Instead of invoking "elm" one now has to use "elm-make".
I kept the 0.13 version of elm around in case someone depends
on it.
Re-apply the build fix from <https://github.com/NixOS/nixpkgs/issues/2089>,
because apparently the underlying issue has not been fixed.
According to <https://github.com/elm-lang/Elm/issues/384>, Elm's release
archive comes with a Setup.hs that cannot compile an Elm release. Duh!
Replacing the custom Setup.hs file with a dummy version fixes this issue.
