一个允许无限数量的并发读取的锁，这些读取永远不会被阻塞。同一时间只能有一个写者访问资源。

示例

use rculock::{RcuLock, RcuGuard};

// Create a new RcuLock protecting a piece of data, in this case a number (u32).
let data: RcuLock<u32> = RcuLock::new(5);
assert_eq!(5, *data.read());
{
    // The data is cloned and handed to the writer
    let mut guard: RcuGuard<u32> = data.write();
    // RcuGuard implements `Deref` and `DerefMut` for easy access to the data.
    *guard = 4;
    // The writer has changed its copy of the data, but the changes
    // have not yet made it back to the master `RcuLock`.
    assert_eq!(5, *data.read());
}
// After the write guard is dropped, the state of the resource
// as the writer sees it is atomically stored back into the master RcuLock.
assert_eq!(4, *data.read());