Better implementation of capture set intersection
Better implementation of capture set intersection

Note: using `accountsFor` criterion would be too pessimistic and therefore
underapproximates elements of intersections, which means soundness would be
lost. We now overapproximate with `mightAccountFor`, at a possible loss of completeness.

lampepfl/dotty,sjrd/dotty,dotty-staging/dotty,lampepfl/dotty,lampepfl/dotty,sjrd/dotty,dotty-staging/dotty,lampepfl/dotty,dotty-staging/dotty,sjrd/dotty,lampepfl/dotty,dotty-staging/dotty,sjrd/dotty,sjrd/dotty,dotty-staging/dotty