Revert "Remove unnecessary overloads, state should always be const."

This reverts commit 9b77c26a24.

As discussed on github, the original code was wrong.

include/boost/fusion/algorithm/iteration/fold_fwd.hpp

@@ -33,6 +33,24 @@ namespace boost { namespace fusion
       , F
     >::type
     fold(Seq const& seq, State const& state, F f);
+
+    template<typename Seq, typename State, typename F>
+    BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
+    inline typename result_of::fold<
+        Seq
+      , State const
+      , F
+    >::type
+    fold(Seq& seq, State& state, F f);
+
+    template<typename Seq, typename State, typename F>
+    BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
+    inline typename result_of::fold<
+        Seq const
+      , State const
+      , F
+    >::type
+    fold(Seq const& seq, State& state, F f);
 }}
 
 #endif