Pillar moves should reuse pillar cache

In a common use case where people configure a pillar swap and a pillar change move, a pillar cache wiill be constructed three times for each step. (Once for the change move, twice for the pillar swap move.) This is possibly a nice performance improvement, if we can get the caches to be shared.

Do note we can only share the caches when the entity selectors and value selectors are the same. Otherwise the pillar caches would represent different data and could not be shared.