Deploy with Haskell on Heroku (cedar-14)

Uses two hooks and a _.profile.d_ script:

- `.halcyon-hooks/sandbox-create`, to install _alex_ and _happy_ in a `build-tools` sub-sandbox within the sandbox layer
- `.halcyon-hooks/app-install`, to include the _idris_ binaries and runtime in the slug
- `.profile.d/try-idris.sh`, to move the _idris_ runtime back in place