Added convenience macro for defining foreign functions from closures

[ysimonson]

Maybe useful enough to bring in? I've been using this macro to define foreign functions from closures.

A couple of examples:

ketos_closure!(scope, "pid", || -> u32 {
    Ok(process::id())
});

ketos_closure!(scope, "setenv", |key: &str, value: &str| -> () {
    env::set_var(key, value);
    Ok(())
});

[murarth]

Looks great. Thanks!