Add utility filesystems

[sminez]

This is a little rough round the edges to start, but I'm adding some utility file systems and proxies that can be used to build larger systems. The RamFs and LocalProxyFs filesystems are concrete, while the HookFs and ReadOnlyFs filesystems wrap another filesystem implementation in order to provide new functionality.

The local_proxy.rs and ramfs.rs examples show how these things look in use:

use ninep::{
    sync::server::Server,
    util::{hook::HookFs, ram::RamFs},
};
use std::env::{args, current_dir};

fn main() {
    let chatty = args().nth(1).as_deref() == Some("--chatty");
    let fs = HookFs::new(RamFs::new(env!("USER"), "group"), move |op| {
        if chatty {
            println!("{op:?}");
        }

        Ok(())
    });

    let s = Server::new(fs);
    let socket_path = current_dir().unwrap().join("ramfs");

    println!("starting ram-fs file server at {}", socket_path.display());
    if s.serve_socket_with_custom_path(socket_path).join().is_err() {
        eprintln!("server thread died");
    }
}