Update structopt example to use clap

[senekor]

inspired by #1848

structopt was merged into clap, and quicli doesn't seem necessary anymore for this example since read_to_string was stabilized.

Maybe the Result<(), Box<dyn Error>> in this version is less nice? But I'd say it's pretty common for quick-and-dirty error handling.

[senekor]

Using anyhow might be nice:

use std::fs::read_to_string;

fn main() -> anyhow::Result<()> {
    let args = Cli::parse();
    read_to_string(&args.file)?
        .lines()
        .take(args.num)
        .for_each(|line| println!("{}", line));
    Ok(())
}

[Manishearth]

Would be nice to get the CLI WG to sign off on this but I think they don't really exist anymore.