[Monitor] Defer constraint checks till `step` boundaries

[ngernest]

Currently, the monitor checks whether a constraint that arises from an assignment DUT.input := a holds eagerly, when it encounters the assignment statement.

However, in the interpreter, assignments can be overwritten. The semantics of overwriting assignments say that if in the same cycle, a concrete (non-X) assignment to DUT.input is followed by a DontCare assignment DUT.input := X, the net-effect at the end of the cycle is that there are no constraints on DUT.input:

DUT.input := 5;
DUT.input := X;
step();       // No constraint on `DUT.input` here
...

However, our monitor does not reflect this behavior as it eagerly checks whether the constraint trace(DUT.input) == 5 holds when it encounters the first assignment DUT.input := 5, and it doesn't take into account the latter DontCare assignment that overwrites it.

To fix this, overwriting assignments should update the monitor's constraints map (i.e. DUT.input := X would cause the constraint DUT.input == 5 to be removed), and we should defer constraint checks to the monitor only when we reach a step() boundary. (This is currently done in bi but not in the monitor.)

This would cause the monitor behavior to be more in-line with the interpreter, which also defers checks for conflicting assignments till step boundaries (#165).