Use `crossbeam-channel` for the channels

[vitiral]

As @burntsushi pointed out (and I had been looking into) chan is not necessarily the best crate for ergonomic channels -- crossbeam-channel is. However, there are several things that I think are unergonomic about both APIs that I would like ergo_sync to improve upon. I am very amenable to discussion on these points.

Panicing on Error

• If you are trying to send but the receiver is disconnected, chan blocks forever and crossbeam-channel returns a RecvError. I would like to panic in the general case.
• I would also like to panic for the reverse case (receiving when there are no senders).

Better macros / syntax

However much I would like to do these things I would not like to wrap the types or add a method.

• [X] Instead I would like to propose the ch! macro (done in this commit with slightly different syntax)

Note: I chose the @ since it is similar to rust's pattern mattching syntax of assigning a matched variable. =<- is another possibility although I would want to use the same pattern in ch! and ch_select!.

// Sending
ch!(send <- /* expression */); // blocks until the expression is sent or panics if there is no receiver.

// Receiving
let x;
ch!(x @<- recv);         // blocks until a value is recieved, stored in x
ch!(let y @<- recv);     // same as above but without the `let` block.
ch!(let mut z @<- recv); // can also specify mutability

• In addition there should be a ch_select! macro which is kind of a combination of ch! above and select_loop!: (nevermind, select_loop will be fine for now)

ch_select!{
    send <- /* expression*/ => { /* event expresssion */ },
    y @<- recv => { /* expresion where you can use y */ },

    // "default" cases"
    disconnected => { ... } // if all cases would disconnect
    would_block() => { ... } // if all cases would block
    timed_out(timeout) => { ... } // if all cases would timeout
}

[vitiral]

the major advantage of ch! being a macro is that panicing uses panic! and therefore contains the line number of failure.