Add setFunctionArgs lib function.

Among other things, this will allow *2nix tools to output plain data
while still being composable with the traditional
callPackage/.override interfaces.

lib/debug.nix

@@ -2,10 +2,10 @@
 
 let
 
-inherit (builtins) trace attrNamesToStr isAttrs isFunction isList isInt
+inherit (builtins) trace attrNamesToStr isAttrs isList isInt
         isString isBool head substring attrNames;
 
-inherit (lib) all id mapAttrsFlatten elem;
+inherit (lib) all id mapAttrsFlatten elem isFunction;
 
 in