dhall: add a static top-level executable for dhall

cc @Gabriel439

Changed files
+2
pkgs
top-level
+2
pkgs/top-level/all-packages.nix
···
cmark = callPackage ../development/libraries/cmark { };
+
dhall = haskell.lib.justStaticExecutables haskellPackages.dhall;
+
dhallToNix = callPackage ../build-support/dhall-to-nix.nix {
inherit (haskellPackages) dhall-nix;
};