Debug representation of opaque structs

[dtolnay]

Currently the handling of opaque structs is inconsistent.

My preference is the one Registration uses, which is the most similar to what #[derive(Debug)] would do.

---

mio/src/poll.rs

Lines 1140 to 1144 in f1c59af

	impl fmt::Debug for Poll {
	fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
	write!(fmt, "Poll")
	}
	}

mio/src/poll.rs

Lines 1525 to 1530 in f1c59af

	impl fmt::Debug for Registration {
	fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
	fmt.debug_struct("Registration")
	.finish()
	}
	}

mio/src/sys/windows/tcp.rs

Lines 587 to 591 in f1c59af

	impl fmt::Debug for TcpStream {
	fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
	"TcpStream { ... }".fmt(f)
	}
	}