dhall: add a static top-level executable for dhall

cc @Gabriel439

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;
   };