Function display 

pub fn display(store: &impl ProcessStore, root_pid: u32) -> String

Render a pstree-style display starting from the given root PID.

use proc_tree::{DefaultStore, display, ProcessStore, ProcessInfo};

let store = DefaultStore::new(0);
store.insert_process(1, ProcessInfo::new("init".into(), "init".into(), "root".into(), 0, 1, 0));
store.insert_process(100, ProcessInfo::new("sshd".into(), "sshd".into(), "root".into(), 1, 100, 0));
store.insert_process(200, ProcessInfo::new("cron".into(), "cron".into(), "root".into(), 1, 200, 0));

let output = display(&store, 1);
assert!(output.starts_with("init"));
assert!(output.contains("sshd"));
assert!(output.contains("cron"));