Use arc_new_cyclic

[tatetian]

The arc_new_cyclic feature has been stablized in Rust. 1.58. Since we are using recently new nightly version, I think we should already have this feature.

This enables creating self-referencing Arc object.

use std::sync::{Arc, Weak};

struct Foo {
    me: Weak<Foo>,
}

impl Foo {
    /// Construct a reference counted Foo.
    fn new() -> Arc<Self> {
        Arc::new_cyclic(|me| Foo {
            me: me.clone(),
        })
    }

    /// Return a reference counted pointer to Self.
    fn me(&self) -> Arc<Self> {
        self.me.upgrade()
    }
}

Since this pattern is so common, NGO even adds a dedicated crate, new-self-ref-arc for exactly this purpose. We should remove the crate and use new_cyclic instead.

Anyone interested in taking this refactoring task?