Support for anonymous pipes

[notpeelz]

Is your feature request related to a problem? Please describe.
It's unclear how anonymous pipes should be used in Tokio. For my use case, I have a RawFd returned from a pipe2() syscall and I want to wrap it with the types in tokio::net::unix::pipe. At the moment, it seems like the only way to do this is to wrap it in a File first:

let file = unsafe { std::fs::File::from_raw_fd(fd) };
let sender = tokio::net::unix::pipe::Sender::from_file(file).unwrap();

Describe the solution you'd like
I'm not sure if wrapping it in a File is the "right" way to do this, but it should at least be documented.

[Darksonn]

We should definitely have a way to construct an anonymous pipe in the API.

[luben]

I found https://crates.io/crates/tokio-pipe works well for my use case. Having it in tokio would be better of course.