Unnecessary trailing Flow::Continue

[uselessgoddess]

At the moment, in any Handler, you need to return Try<Output = ()>. This is cool and allows interrupting operations in errors. But it inflates the code:

// one operation handler
|link| {
    worker.work(link);
    Continue // <- :(
}

But everyone wants to:

|link| worker.work(link)

I see two ways

---

1. Inspired by std::Termination create Termination-like trait

It might look something like this:

trait HandlerResult {
    type Try: Try<Output = ()>;

    fn try_it(self) -> Self::Try;
}

// impl for ()
// impl for all Try

2. Discard `Handlers' outside the internal code (track it https://github.com/linksplatform/doublets-rs/issues/3)

Use $OP_iter so each_iter and others example above:

.each_iter(...).for_each(|link| worker.work(link))

What do you think about this?

In particular @Konard