`#![feature(..)]` outside of crate root only warns and should be louder

[estebank]

When a feature attribute is declared anywhere but the crate root, we currently emit an unused attribute warning:

fn main() {
    #![feature(default_field_values)]
    let () = 1;
}

This has caused issues in the past with people not realizing that the attribute wasn't set in the right place. It should probably be its own error/deny-by-default lint, specifically for feature.