Use `span_suggestion` instead of manually building suggestions

[Manishearth]

http://manishearth.github.io/rust-internals-docs/rustc/session/struct.Session.html#method.span_suggestion

See, for example, the suggestion you get for http://is.gd/5k86CL, it's built by running span_suggestion with format!("move {}", snippet_for_closure).

[sanxiyn]

Manishearth/compiletest-rs#30 is merged. Waiting on the new version to update dependency.

[oli-obk]

I think we're doing this everywhere now.