Function deduplicate_arg_names 

fn deduplicate_arg_names(arg_names: &mut [String])

Makes duplicate argument names unique by appending incrementing numbers.

let mut names: Vec<String> =
    vec!["foo".into(), "foo".into(), "bar".into(), "baz".into(), "bar".into()];
deduplicate_arg_names(&mut names);
let expected: Vec<String> =
    vec!["foo_1".into(), "foo_2".into(), "bar_1".into(), "baz".into(), "bar_2".into()];
assert_eq!(names, expected);