Converted exception_ptr from a shared_ptr typedef to a stand-alone class to allow simpler declaration of namespace boost { class exception_ptr; }

[SVN r68202]

libs/exception

@@ -1 +1 @@
-Subproject commit c210c38dae64acf9dc7c26b5e013ed3f289a0981
+Subproject commit 6b0bfad6bfffd40f0744071c53ab69bb6a0f0d19