Add try_recv() for non-blocking event queue reads

[RobDavenport]

Discussed in https://github.com/lemunozm/message-io/issues/46.

I tried to run rustfmt but ran into some issues with the .rustfmt.toml file. Probably a better idea to wait until that is fixed before merging this.

I also wrote some unit tests to cover some of the common use cases. Would appreciate some extra 👀 on it to make sure I've done it correctly. I'm sure there's a more elegant way of doing this but seems to serve as a solid foundation. Either way, feedback/comments appreciated!

Discussion points:

• Should this be named try_recv or try_receive?
• Does the comment look OK?
• I really wanted to avoid copying the instant but was running into multiple mutable borrow issues for self.timers

[lemunozm]

Sorry, I forgot the "instant" problem, let me check...

[lemunozm]

The problem is that the remove function takes the borrow of the instant that wants to remove. I saw that there is a function that allows performing exactly this but is nightly :(

Anyway, copy the instant is more or less as copy a number, so I think this copy is depreciable or maybe even optimized by the compiler.

As a minor minor really minor fix, you could move the copy inside the if and only copy when the condition was true:

if *next_instant.0 <= Instant::now() {
    let next_instant = *next_instant.0;
    return self.timers.remove(&next_instant);
}

But I think that for things like this the compiler will move it by us.

[lemunozm]

Regarding .rustfmt.toml. It has features of fmt nightly. I usually program in nightly to get the advantage of things like this but using always stable features.

[RobDavenport]

Thanks for the feedback - except an update within a short while while I make the changes!

[lemunozm]

Thanks, @RobDavenport for the contribution 😃 !!