One-shot value exhange between threads.
The consumer thread may await the producer thread Both the producer and consumer are single use

Just a wrapper around a std::mpsc channel with an API that makes it impossible to use more than once

# Example

```
let (send, recv) = promissory::promissory();
std::thread::spawn(move || send.fulfill(42u32));
assert_eq!(42, recv.await_value()?;
```