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/view/detail/strictest_traversal.hpp

@@ -59,7 +59,7 @@ namespace boost { namespace fusion
             // never called, but needed for decltype-based result_of (C++0x)
 #ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
             template<typename StrictestSoFar, typename Next>
-            BOOST_FUSION_GPU_ENABLED
+            BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
             typename result<strictest_traversal_impl(StrictestSoFar, Next)>::type
             operator()(StrictestSoFar&&, Next&&) const;
 #endif