WeakRef requires rescuing RefError to avoid race condition

There's an race condition in the implied usage of WeakRef.

The API only has weakref_alive?, and then delegated access to the referenced object. But the delegated access to the object cannot be protected by weakref_alive? since GC may occur between the check and the usage.

This means we basically always have to check for RefError, which basically makes weakref_alive? useless if we want to actually potentially use the object.

WeakMap usage is discouraged, leaving us with needing to add this functionality to WeakRef, which may break if WeakRef implementation changes (i.e., there is no good solution for this).

This is discussed at length here:

https://stackoverflow.com/questions/69185508/ruby-weakref-has-implicit-race-condition

I would recommend an addition to the API that will safely return a (non-weak) object if it's alive, or else nil, and obviously it's up to the user to realize that this will stop GC from happening on that object while they hold it.

Start with the WeakRef API, especially weakref_alive?, delegated access, RefError handling, and the discouraged WeakMap alternative. Read the linked Stack Overflow discussion for the race condition context; done means agreeing on and implementing an API that safely returns a strong referenced object when alive or nil otherwise.