Add support for retrieving the arity of a function type without partial

specialization. Document and test this


[SVN r14694]

libs/type_traits

@@ -1 +1 @@
-Subproject commit 8f8e96630c1effc3617827779730940f05791e34
+Subproject commit 1ba525e29deb6f0c56b1e55c796aab3a5ff7c8a6