Idea: improved Select struct

[ghost]

The current Select stores closures for firing cases. This is slow and interacts poorly with the borrow checker.

I think we could have interface like this:

let mut sel: Select<'_> = Select::new();
let id1 = sel.add_send(&s);
let id2 = sel.add_recv(&r);
let id = sel.wait();

if id == id1 {
    let res = sel.finish_send(s, msg);
} else if id == id2 {
    let res = sel.finish_recv(r);
}

Functions finish_send and finish_recv would panic if wrong sender or receiver is passed. Select stores a Token internally that is used for the finishing function call.

Also, we could probably support reusing sel in multiple selections.

As further simplification, we could also:

• Remove the codegen in select! and instead "compile" the macro invocation down to Select.
• Remove support for sending into iterators of receivers/senders in select! because the syntax odd, it is a niche use case, and can be done using Select anyway.