Mode is not properly updated on push/pop

[bclement-ocp]

Making an issue so we don't forget:

(set-option :produce-models true)
(set-logic ALL)
(declare-const x Int)
(assert (<= x 10))
(push 1)
(check-sat)
(pop 1)
(get-model)

should be rejected (originally reported by @Halbaroth)

This is because we don't update the DO.Mode in push and pop.