lint unnecessary .into_iter()

[zackmdavis]

for loops implicitly call .into_iter() on that which is being iterated over, but if someone didn't know this, they might have written .into_iter() themselves (this is not merely a hypothetical scenario), which is surely bad style.

Example of code that should get a warning (but does not as of Clippy v. 0.0.79):

fn main() {
    let my_vec = vec![1, 2, 3, 4];
    for i in my_vec.into_iter() {
        println!("{}", i);
    }
}

(could be just for i in my_vec {)

I'm guessing the implementation would probably be pretty similar to that of the existing explicit_iter_loop lint?