Suggest `for` loop over `while let` when looping over iterator

[frewsxcv]

If someone does this:

#![feature(plugin)]

#![plugin(clippy)]


fn some_function() {
    let mut some_iterator = 1..20;
    while let Some(x) = some_iterator.next() {
        println!("{}", x);
    }
}

suggest doing this:

#![feature(plugin)]

#![plugin(clippy)]


fn some_function() {
    let mut some_iterator = 1..20;
    for x in some_iterator {
        println!("{}", x);
    }
}

[fhartwig]

I'll give this a go.

[Manishearth]

Let us know if you need help!