feat(kani): unit proof for add_used method (sec 2.8.22)

Verify that the add_used method updates the queue on success and does not update on failure.
For this (and subsequent proofs), implement a stub region that supports stateful proofs.
We add a unit proof to make sure that the stub region itself is working as intended.

Signed-off-by: Siddharth Priya <[email protected]>

Author: priyasiddharth