Merge new changes to unordered & hash.

- Unordered tests can run lightweight test or Boost.Test (at least
   theoretically).
 - Workaround Open BSD's incorrect numeric_limits.
 - Move the hash extensions in their own file.
 - Various small improvements to the unordered docs.
 - Fix some unordered examples.

Merged revisions 43117-43837 via svnmerge from 
https://svn.boost.org/svn/boost/branches/unordered/trunk


[SVN r43838]

test/unordered/insert_tests.cpp

@@ -5,7 +5,7 @@
 
 #include <boost/unordered_set.hpp>
 #include <boost/unordered_map.hpp>
-#include <boost/detail/lightweight_test.hpp>
+#include "../helpers/test.hpp"
 #include <boost/next_prior.hpp>
 #include "../objects/test.hpp"
 #include "../helpers/random_values.hpp"