`TailQueue` was implemented as a doubly-linked list, but named after an
abstract data type. This was inconsistent with `SinglyLinkedList`, which
can be used to implement an abstract data type, but is still named after
the implementation. Renaming `TailQueue` to `DoublyLinkedList` improves
consistency between the two type names, and should help discoverability.
`TailQueue` is now a deprecated alias of `DoublyLinkedList`.
Related to issues #1629 and #8233.
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