rustc: Explicitly mention type params with missing `'static` bounds

This is inspired by the [question](http://www.reddit.com/r/rust/comments/1yy57k/unsolved_question_from_irc/) (re-)posted to /r/rust. The error message in this question correctly states that one should add `'static` to the trait bounds, but does not state which trait bounds. This PR makes that explicit by appending two words.

This also renames `check_durable` to `check_static` and removes the outdated comment as a cleanup.