Fixed points of PWMap

[Kalashnikovni]

As noted in iss-70, the detection of fixed points of a pw is buggy. I intend to correctly implement a fixedPoints function for PWMaps.
Ps. I mention iss-70, but I'm almost certain that there same other parts of the algorithms where this idea is wrongly replicated.

[Kalashnikovni]

I will also implement a basic UnorderedSet::compact() function that currently returns the same set.

[Kalashnikovni]

As noted in pr-53, MDIs that should be compacted aren't compacted. The MultiDimInter::compact() implementation needs to be updated.