Use forwarding to get SFINAE effect in some common use cases.

Rename detail::result_of to detail::result_of_impl to avoid surprises
when result_of is used from within boost::detail.


[SVN r33981]

libs/utility

@@ -1 +1 @@
-Subproject commit 271ea9e901bdbb8110ab200d8584da37a7864042
+Subproject commit 7ac180ed548c2b7abb8077f35fa8b857f12b8487