Add support for Date / Time / TimeZone

[Gabriella439]

… as standardized in dhall-lang/dhall-lang#1191

[Gabriella439]

This PR also highlights an issue with our GitHub action CI: it doesn't exercise changes to dhall that affect downstream packages. For example, if the change to the dhall package breaks the dhall-json package then the GitHub CI will still pass because the dhall-json build is building against the latest dhall from Hackage, not the local dhall package.

[sjakobi]

For example, if the change to the dhall package breaks the dhall-json package then the GitHub CI will still pass because the dhall-json build is building against the latest dhall from Hackage, not the local dhall package.

@Gabriel439 I don't understand what problem you're describing here. If the dhall-json build would attempt to use the latest dhall from Hackage, surely it should fail because Date, DateTime and the other new exports would be out of scope. CI, including the dhall-json jobs, is green though!

[Gabriella439]

@sjakobi: You're right. I misdiagnosed the problem. Downstream dhall-* packages are building against the local dhall package