[Civl] Introduction of pure Assert action

[shazqadeer]

Call to pure action is treated specially on the disappearing layer of the caller. At lower layers, the gate is asserted during invariant checking. But at the disappearing layer, the gate is assumed during invariant checking and asserted during refinement checking. This PR introduces a special pure action Assert which can be used as an alternative to the assert statement with the behavior explained above. As a result, the semantics of assert statements becomes simple; they are asserted during invariant checking and assumed during refinement checking.