Publicise types/add #[allow(visible_private_types)] to a variety of places.

There's a lot of these types in the compiler libraries, and a few of the
older or private stdlib ones. Some types are obviously meant to be
public, others not so much.

src/libnative/io/timer_timerfd.rs

@@ -44,6 +44,7 @@ pub struct Timer {
     priv on_worker: bool,
 }
 
+#[allow(visible_private_types)]
 pub enum Req {
     NewTimer(libc::c_int, Chan<()>, bool, imp::itimerspec),
     RemoveTimer(libc::c_int, Chan<()>),