std: Rename Chan/Port types and constructor
std: Rename Chan/Port types and constructor

* Chan<T> => Sender<T>
* Port<T> => Receiver<T>
* Chan::new() => channel()
* constructor returns (Sender, Receiver) instead of (Receiver, Sender)
* local variables named `port` renamed to `rx`
* local variables named `chan` renamed to `tx`

Closes #11765

rust-lang/rust.vim