Add "nix add-to-store" command

src/libutil/args.hh

@@ -81,15 +81,16 @@ public:
         FlagMaker & handler(std::function<void(Strings)> handler) { flag->handler = handler; return *this; };
         FlagMaker & category(const std::string & s) { flag->category = s; return *this; };
 
-        FlagMaker & dest(std::string * dest) {
-            assert(flag->arity == 1);
+        template<class T>
+        FlagMaker & dest(T * dest) {
+            flag->arity = 1;
             flag->handler = [=](Strings ss) { *dest = ss.front(); };
             return *this;
         };
 
         template<class T>
         FlagMaker & set(T * dest, const T & val) {
-            assert(flag->arity == 0);
+            flag->arity = 1;
             flag->handler = [=](Strings ss) { *dest = val; };
             return *this;
         };