is_abandoned() is broken since Rust 1.74.0

[mgeier]

We have previously been relying on the undocumented synchronization in Arc::strong_count().
This has been removed in rust-lang/rust#115546, which broke is_abandoned().

There have been others who were surprised by the change: rust-lang/rust#117485

For reference, this is the documentation: https://docs.rs/rtrb/0.3.0/rtrb/struct.Producer.html#method.is_abandoned

[mgeier]

I added some documentation in #122 and #124.