More constexpr and noexcept support.

Note 1: Forwarding functions are specified as a C++14 constexpr since
std::forward is not a constexpr within C++11.

Note 2: Though I'm not sure why it doesn't compile, some declarations
are specified as a C++14 constexpr or non-constexpr.

Note 3: Boost.Tuple adaptation and TR1-based tuple implementations are
not constexpr.

include/boost/fusion/adapted/array/at_impl.hpp

@@ -27,7 +27,7 @@ namespace boost { namespace fusion { namespace extension
                 add_reference<typename remove_extent<Seq>::type>::type
             type;
 
-            BOOST_FUSION_GPU_ENABLED
+            BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
             static type
             call(Seq& seq)
             {