These aren't fully static executables (they still have some
`/nix/store` references), but they at least compile Haskell dependencies
statically. That means that they can be `nix-env --install`ed side-by-side
with other Haskell executables, which would otherwise conflict with an
error like this one:
```
error: packages '/nix/store/hrxnlwlsiw5jjjkq5v6ihcwb0shx4fga-dhall-1.20.1/lib/li
nks/libHSbasement-0.0.8-8QjArDsw3GWCcbHE5iqtz3-ghc8.4.3.dylib' and '/nix/store/d
2y5373anwf1q3h86ar3lljk11k1lq0h-dhall-json-1.2.6/lib/links/libHSbasement-0.0.8-8
QjArDsw3GWCcbHE5iqtz3-ghc8.4.3.dylib' have the same priority 5; use 'nix-env --s
et-flag priority NUMBER INSTALLED_PKGNAME' to change the priority of one of the
conflicting packages (0 being the highest priority)
```
The motivation for this change is:
* To catch build failures in downstream packages whenever we make a breaking
change to the `dhall` API
* To reduce the amount of work I need in order to cut a release for all of
these packages
* To better share Nix/CI-related logic between the projects
Note that I have not yet migrated `dhall-nix` in. I'm waiting for
https://github.com/dhall-lang/dhall-nix/issues/17 to be fixed since
`dhall-nix` is incompatible with later versions of `megaparsec` due to
`hnix`.
Gabriel Gonzalez