Function display 

pub fn display(tree: &impl TreeStore, root_pid: u32) -> String

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

use proc_tree::{DefaultTree, display, TreeStore, PidNode};

let tree = DefaultTree::new(100, 0);
tree.insert_node(1, PidNode { ppid: 0, cmd: "init".into() });
tree.insert_node(100, PidNode { ppid: 1, cmd: "sshd".into() });
tree.insert_node(200, PidNode { ppid: 1, cmd: "cron".into() });

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