[racket] [racket-dev] Implementing contracts for async channels

One comment. The contract combinators are curried so that you can do
work on the partial applications. So don't write this:

(define ((((ho-val-first-projection
impersonate/chaperone-async-channel) ctc) blame) val)

instead try to do some work earlier, when you first can. (The most
important thing is to minimize the work done after you get the 'val'
argument.)

Robby