Stop using public newtype structs to wrap private types

[sfackler]

std::rt uses this a lot, for example:

pub struct TcpStream(~RtioTcpStreamObject);

This doesn't properly encapsulate the private type inside, since client code can dereference the TcpStream to get at it. According to #5489, newtype structs aren't intended to be able to hide their internals. The easiest solution is to probably switch to single-element structs, e.g.:

pub struct TcpStream {
    priv inner: ~RtioTcpStreamObject
}