Fix UB in pool module when using ArcBlock<T> and BoxBlock<T>

[Mortano]

This fixes UB when using the ArcBlock<T> and BoxBlock<T> types together with the Stack type, as it occurs in the ArcPoolImpl and BoxPoolImpl types. The underlying cause for the UB was a wrongly initialized UnionNode, as was discussed in #605 . This also required that the functions of the Node trait are unsafe, but this should not be a breaking API change as that trait is never exported from the pool module

[sgued]

LGTM

Thanks a lot for the audit of the unsafe code. Already yielding results!