hpack-dhall-0.5.2: hpack's dhalling

Work with hpack's top-level fields in a Dhall record with the following executables;

• with dhall-hpack-cabal write the .cabal for a .dhall package description.
• with dhall-hpack-dhall show the package description expression, with imports resolved.
• with dhall-hpack-json show the package description as JSON.
• with dhall-hpack-yaml show the package description as YAML.