Separate stderr and compiler output

[tgross35]

Program stderr kind of gets blended in with the compiler output:

The output "middle" is easy to miss since there's no visual separation from the compiler message. It would be nice to have a separate section "Compiler Output" to go with the existing "Standard Error" and "Standard Output" sections.

Making the output sections collapsible under the heading might be good as well, if having an extra separator would consume too much extra vertical space.

(snippet for example:)

fn main() {
    let x = 10u32; // trigger unused
    println!("start");
    eprintln!("middle");
    println!("end");
}