A unique id for file descriptions. While we could use the address, considering that
is definitely unique, the address would expose interpreter internal state when used
for sorting things. So instead we generate a unique id per file description is the name
for all duplicates and is never reused.
A helper trait to indirectly allow downcasting on Rc<FdIdWith<dyn _>>.
Ideally we’d just add a FdIdWith<Self>: Any bound to the FileDescription trait,
but that does not allow upcasting.