Simulator wastes a lot of resources computing states that won't be used

[bugarela]

When there is an any, Quint simulator will do the following:

1. Execute all actions and save the result and the state after that action
2. Filter out actions for which the result is false (i.e. pre-conditions not satisfied)
3. From the remaining actions, randomly pick one
4. Take the state from the picked action to be the next state in the simulation and proceed

This means that we can compute a bunch of stuff in 1 that will never be used.

I need to think if this won't affect the probability distribution too much, but an alternative approach would be:

1. pick an action
2. execute it. If the result is false, try the next one
3. if we get back to the picked action, the result of any is false

This would also mean we would make a rand() call even on deterministic anys (that is, with disjoint pre-conditions). But I think is would be a much smaller problem than the performance issue.