Move Build.hs (which builds elm-runtime.js) into a cabal build step (in compiler folder)
The elm exec and runtime should never get out of sync
Having a separate Build.hs file also causes problems when using cabal-dev
Moving elm-runtime.js into the dist/ folder also shows that it's generated (and cabal clean will remove it)
Similar for types.json (although whether this should be part of the build step, I don't know)
