chan is end-of-life; use crossbeam-channel instead

**`chan` has reached its end-of-life and is now deprecated.**

The intended successor of this crate is
[`crossbeam-channel`](https://github.com/crossbeam-rs/crossbeam/tree/master/crossbeam-channel).
Its API is strikingly similar, but comes with a much better `select!` macro,
better performance, a better test suite and an all-around better
implementation.