Half-reified propagators

[EmirDe]

We can create a half-reified version of propagators in a generic way. I think there was space to improve it so that propagation goes both ways.

[maartenflippo]

Assume propagator $P$ is half-reified by $r$: $r \rightarrow P$. If $P$ implements Propagator::detect_inconsistency, then $r$ will be propagated to false whenever $P$ detects a conflict.

At the moment, at least the linear inequality propagator implements detect_inconsistency, which is the most important one for FlatZinc.

[EmirDe]

I was thinking it would be nice to have a generic reified propagator that works out of the box. Essentially, the reified propagator maintains its own assignments object relevant for its internal propagator. For the propagator, it would like the usual case, it would not be aware that it is within a reified propagator. It would also make unwatching propagators easier if we ever get to the point that we deactive propagators during search.