Add Natural/showHex builtin

[kukimik]

Implementation of dhall-lang/dhall-lang#1351.

Mostly copy-pasted from 3aadd57.

• Unfortunately I don't know nix and I wasn't able to add a proper nix interpretation of showHex in dhall-nix/src/Dhall/Nix.hs. I've found something called lib.trivial.toHexString, but this probably can't be used directly.
• I don't know if (and how/at which moment) I should update the dhall/dhall-lang subrepo.

[kukimik]

Unfortunately I don't know nix and I wasn't able to add a proper nix interpretation of showHex in dhall-nix/src/Dhall/Nix.hs. I've found something called lib.trivial.toHexString, but this probably can't be used directly.

I've learnt enough nix to write this function down 🎉 (which turned out to be rather easy), now I just need to find a moment to hnix-ify it.

[kukimik]

I added a nix translation of Natural/showHex. I don't really like the elemAt approach; I'd rather do it like this, but I didn't have the time to dig through hnix's Antiquotes. Perhaps the current solution is good enough.

[Gabriella439]

Just FYI: I'm probably going to cut a release of the Haskell implementation before merging this and the corresponding dhall-lang PR, just because I'm a bit overdue on cutting a release

[kukimik]

Closing the PR for now; see the discussion in dhall-lang/dhall-lang#1351.