Represents a composite of cancelables that are stacked,
so you can push a new reference, or pop an existing one and
when it gets canceled, then the whole stack gets canceled.
Similar in spirit with CompositeCancelable, except that
you can only pull out references in a FIFO fashion.
Used in the implementation of monix.eval.Task.
Cancels the unit of work represented by this reference.
Guaranteed idempotency - calling it multiple times should have the
same side-effect as calling it only once. Implementations
of this method should also be thread-safe.
true in case this cancelable hasn't been canceled,
or false otherwise.