Derive Clone for Command and StdioContainer

[japaric]

Allows use cases like this one:

use std::io::Command;

fn main() {
    let mut cmd = Command::new("ls");
    cmd.arg("-l");

    for &dir in ["a", "b", "c"].iter() {
        println!("{}", cmd.clone().arg(dir));
    }
}

Output:

ls '-l' 'a'
ls '-l' 'b'
ls '-l' 'c'

Without the clone(), you'll end up with:

ls '-l' 'a'
ls '-l' 'a' 'b'
ls '-l' 'a' 'b' 'c'

cc #15294

[huonw]

(FWIW, it would be neat if TaskBuilder could similar treatment, but the internals look too complicated for that to be possible now.)

[aturon]

@huonw Note that TaskBuilder is set up as a "consuming" builder (the spawn method takes self) because of the inability to clone a Writer in general; that also stands in the way of supporting Clone for TaskBuilder itself. I don't see any way around that, sadly.