This causes text literals to be formatted as multi-line strings
whenever they contain at least one newline and at least one non-newline
character. "Spacers" like `"\n\n"` continue be formatted as single-line
strings. If the heuristic turns out to be too eager to choose a
multi-line layout, we can refine it later.
This partially addresses #1496.
Also
* update some variable names
* use 80-column "smart" layout consistently
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`.
