new lint: suggest using iterator instead of casting iterator to vector end using vector, in simple cases

code snippet:

fn main() {
    let x = String::from("this-is-a-string");
    let y: Vec<&str> = x.split("-").collect(); // vec
    for i in y {
        println!("{}", i);
    }
}

if we know that we use x only for iterating over the items once, we can ommit the transformation to a vector completely which is more idiomatic and can be slightly faster I think:

 fn main() {
    let x = String::from("this-is-a-string");
    let mut y = x.split("-"); // iterator
    for i in y {
        println!("{}", i);
    }
}

No source file, test, or entry point is named in the issue. Start with the Rust examples and inspect existing Clippy lint patterns; done means the simple collect-to-vector case is recognized and an iterator-based suggestion is produced without covering cases outside the stated scope.