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/default.nix

@@ -51,12 +51,12 @@ let
 
     inherit (builtins) add addErrorContext attrNames
       concatLists deepSeq elem elemAt filter genericClosure genList
-      getAttr hasAttr head isAttrs isBool isFunction isInt isList
+      getAttr hasAttr head isAttrs isBool isInt isList
       isString length lessThan listToAttrs pathExists readFile
       replaceStrings seq stringLength sub substring tail;
     inherit (trivial) id const concat or and boolToString mergeAttrs
       flip mapNullable inNixShell min max importJSON warn info
-      nixpkgsVersion mod;
+      nixpkgsVersion mod functionArgs setFunctionArgs isFunction;
 
     inherit (fixedPoints) fix fix' extends composeExtensions
       makeExtensible makeExtensibleWithCustomName;