Add conditional trait.

Update and regenerate docs.
Update type_traits.hpp and add trivial test case for it.

[SVN r65575]

libs/type_traits

@@ -1 +1 @@
-Subproject commit edd98db5fbdd06dd932f5ea7ba895b074a9993ff
+Subproject commit c77a72a7cc4cde12b2f86f9dd4d52ff3cf0cb2e3