Improve constexpr supports

- fusion::fold (and other variations) now satisfy C++11 constexpr requirements.
 - will fix [Test output: BP x86_64 C++11 - fusion - deque_hash / gcc-4.8.3~c11](http://www.boost.org/development/tests/develop/developer/output/BP%20x86_64%20C++11-boost-bin-v2-libs-fusion-test-deque_hash-test-gcc-4-8-3~c11-debug-debug-symbols-off.html) and many other failures ...
- fusion::swap now uses BOOST_CXX14_CONSTEXPR instead of BOOST_CONSTEXPR.