net: implement AsRef<Self> for TcpStream and UnixStream

[aatifsyed]

Motivation

I'm writing a wrapper that I want to work for OwnedReadHalf and TcpStream.
OwnedReadHalf: AsRef<TcpStream> already, so this helps.

Solution

TcpStream: AsRef<Self>

[Darksonn]

Hmm. Initially I was going to say no to this, but it seems that the stdlib documentation says that libraries should add this kind of impl for AsRef, so I'm going to accept it.

However, please add the AsRef impl to other types with owned read/write halves such as UnixStream too.

[aatifsyed]

Thanks for your review, I've made the requested changes :)

[Darksonn]

Thanks.