Suggestion: make `trace`, `fixpoints`, and `trapspaces` all return the data structure

[jonrkarr]

All 3 methods return data in a similar structure. However, trace returns as iterable whereas fixpoints returns a list.

[aurelien-naldi]

The return type reflects some parts of the implementation but maybe we could make them all return an iterable.

Trace returns an iterable as the computation of the successor is only performed on demand.

Fixpoints where initially computed using decision diagrams, meaning that all results must be computed at the same time, allowing us to return a list (which is an iterable). We also have an ASP-based implementation which we could use to compute the next result on demand and avoid blocking when there are many solutions.

We could return iterators for all of them, which would let us for example build a list of the first 100 results.