rustdoc no longer shows trait implementations in the documentation

[thestinger]

This was caused by 387aa36 - rustdoc has an is_visible function that checks to see if items have ast::public visibility and trait implementations don't. It needs to check if something is a trait implementation, and if so make it visible if the trait is public.

Example: http://static.rust-lang.org/doc/std/treemap.html (no Map or Set impl shown)

[thestinger]

Fixed by deeeaf0, which shows all trait implementations. Opened #5416 to replace this.