Add empty method to iterators

[jdm]

Sometimes it's desirable to check whether an iterator will return any elements without using len, which is (presumably) O(N). You can probably use some_iterator.any(|_| true), but it would be nicer to hide that behind a nicer API I think.

[thestinger]

There's also the option of just calling next().is_some().

[jdm]

That, too, is less pleasant than a new method.

[mitsuhiko]

Please don't do this. That means the iterator advancing would no longer be atomic which currently is a very desirable property.

You can already make an iterator peekable so that work for many cases.

[mitsuhiko]

Note that .next() is undefined once it returned None once. So defining .empty() as alias to .next().is_some() sounds like a really corner case that is easy to misuse. This pattern would only be safe on a .fuse()d iterator.