TreeExplorer: fix spurious copy of iterator (argument) on functor invocation

...since our iterators *always* yield a reference to the exposed element,
we can *always* get that referency into the nested yield to obtain the value

src/lib/iter-tree-explorer.hpp

@@ -354,7 +354,7 @@ namespace lib {
         template<class IT>
         struct ArgAccessor<IT,   enable_if<std::is_convertible<typename IT::value_type, Arg>>>
           {
-            static auto build() { return [](auto iter) { return *iter; }; }
+            static auto build() { return [](auto& iter) { return *iter; }; }
           };