Allow to generare quoted scalars if needed via providing a custom encode
options to Data.Yaml.encodeWith. So far two corner cases from yaml
itself (an empty scalar, and special strings) are omitted in the
implementation.
... as standardized in https://github.com/dhall-lang/dhall-lang/pull/438
This also adds `dhall-json` support for empty alternatives
In particular, this translates empty alternatives to strings encoding the alternative name
```haskell
-- ./example.dhall
let Role = < Wizard | Fighter | Rogue >
in [ Role.Wizard, Role.Fighter ]
```
```
$ dhall-to-json <<< './example.dhall'
["Wizard","Fighter"]
```
... as proposed in https://github.com/dhall-lang/dhall-kubernetes/issues/46#issuecomment-468530601
`--omitEmpty` is the same as `--omitNull` except it also omits fields that
are empty records. To be precise, it omits fields whose transitive fields
are all `null` (and an empty record is a special case of a record whose
transitive fields are all `null` since it is vacuously true when there are
no fields).
This allows Dhall configurations that target YAML to avoid having to nest
sub-records inside of an `Optional` value. Omitting unnecessary `Optional`
layers reduces the number of default values that the configuration format
needs to thread around.
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`.
