Unconditionally evaluate hook.extra as a function (#61)

In NixOS/nixpkgs#386208, function arguments are preserved in some
circumstances. This caused issues such as divnix/std#399. Ultimately,
it comes down to the `builtins.isFunction` check, which does not detect
functors.

With this patch, `hook.extra` is coerced to a function unconditionally.
If it is a constant, any arguments are ignored, so behaviour does not
change.

Author: korrat

lib/make.nix

@@ -37,10 +37,7 @@ in
         hook =
           userRequest.hook
           // {
-            extra =
-              if builtins.isFunction userRequest.hook.extra
-              then userRequest.hook.extra userRequest.data
-              else userRequest.hook.extra;
+            extra = (pkgs.lib.toFunction userRequest.hook.extra) userRequest.data;
           };
       };