Add mpsc select

Rust has nice implementations of channels, but is missing an implementation of select (i.e. wait on multiple channels), which severely limits their usefulness.

The usual solution is to give up on std::sync::mpsc::channel entirely and use crossbeam-channel instead. That's rather unfortunate - it's like providing a file API, but there's no write functionality so you have to use a third party crate for all file operations instead.

Years ago there was an unstable implementation, but it was removed in 2019 without ever being stabilised (the implementation was apparently unsafe and had other issues - see #27800).

However since 2022, std::sync::mpsc::channel has been implemented in terms of crossbeam-channel, which makes it even more weird that you still have to use crossbeam-channel, and also presumably means that importing crossbeam-channel's select implementation is much easier.

Could we do that?