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/nview/detail/advance_impl.hpp

@@ -12,7 +12,7 @@
 #include <boost/mpl/advance.hpp>
 #include <boost/fusion/iterator/advance.hpp>
 
-namespace boost { namespace fusion 
+namespace boost { namespace fusion
 {
     struct nview_iterator_tag;
 
@@ -36,7 +36,7 @@ namespace boost { namespace fusion
                 typedef nview_iterator<sequence_type, 
                     typename mpl::advance<iterator_type, Dist>::type> type;
 
-                BOOST_FUSION_GPU_ENABLED
+                BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
                 static type
                 call(Iterator const& i)
                 {

include/boost/fusion/view/nview/detail/at_impl.hpp

@@ -11,7 +11,7 @@
 #include <boost/fusion/support/config.hpp>
 #include <boost/fusion/sequence/intrinsic/at.hpp>
 
-namespace boost { namespace fusion 
+namespace boost { namespace fusion
 {
     struct nview_tag;
 
@@ -32,8 +32,8 @@ namespace boost { namespace fusion
                 typedef typename result_of::at<index_type, N>::type index;
                 typedef typename result_of::at<sequence_type, index>::type type;
 
-                BOOST_FUSION_GPU_ENABLED
-                static type 
+                BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
+                static type
                 call(Sequence& seq)
                 {
                     return fusion::at<index>(seq.seq);

include/boost/fusion/view/nview/detail/begin_impl.hpp

@@ -35,7 +35,7 @@ namespace boost { namespace fusion
                 typedef nview_iterator<Sequence, 
                     typename mpl::begin<index_type>::type> type;
 
-                BOOST_FUSION_GPU_ENABLED
+                BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
                 static type call(Sequence& s)
                 {
                     return type(s);

include/boost/fusion/view/nview/detail/deref_impl.hpp

@@ -34,7 +34,7 @@ namespace boost { namespace fusion
                 typedef typename result_of::at<
                     typename sequence_type::sequence_type, index>::type type;
 
-                BOOST_FUSION_GPU_ENABLED
+                BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
                 static type call(Iterator const& i)
                 {
                     return at<index>(i.seq.seq);

include/boost/fusion/view/nview/detail/distance_impl.hpp

@@ -31,7 +31,7 @@ namespace boost { namespace fusion
                     typename First::first_type, typename Last::first_type
                 >::type type;
 
-                BOOST_FUSION_GPU_ENABLED
+                BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
                 static type
                 call(First const& /*first*/, Last const& /*last*/)
                 {

include/boost/fusion/view/nview/detail/end_impl.hpp

@@ -36,7 +36,7 @@ namespace boost { namespace fusion
                 typedef nview_iterator<Sequence, 
                     typename mpl::end<index_type>::type> type;
 
-                BOOST_FUSION_GPU_ENABLED
+                BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
                 static type call(Sequence& s)
                 {
                     return type(s);

include/boost/fusion/view/nview/detail/next_impl.hpp

@@ -35,7 +35,7 @@ namespace boost { namespace fusion
                 typedef nview_iterator<sequence_type,
                     typename mpl::next<first_type>::type> type;
 
-                BOOST_FUSION_GPU_ENABLED
+                BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
                 static type
                 call(Iterator const& i)
                 {

include/boost/fusion/view/nview/detail/nview_impl.hpp

@@ -63,7 +63,7 @@ namespace boost { namespace fusion { namespace result_of
 namespace boost { namespace fusion
 {
     template<BOOST_PP_ENUM_PARAMS(N, int I), typename Sequence>
-    BOOST_FUSION_GPU_ENABLED
+    BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
     inline nview<Sequence, mpl::vector_c<int, BOOST_PP_ENUM_PARAMS(N, I)> >
     as_nview(Sequence& s)
     {

include/boost/fusion/view/nview/detail/prior_impl.hpp

@@ -35,7 +35,7 @@ namespace boost { namespace fusion
                 typedef nview_iterator<sequence_type,
                     typename mpl::prior<first_type>::type> type;
 
-                BOOST_FUSION_GPU_ENABLED
+                BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
                 static type
                 call(Iterator const& i)
                 {