Datalog stratifier is too slow

[daghovland]

Describe the bug
The stratification program has very bad performance, and currently breaks down already on 248 rules

To Reproduce
Steps to reproduce the behavior:

1. Enable the test TestImfDatalog in StratifierTests.fs
2. This does not terminate, at least in a couple of minutes

Expected behavior
Only a few seconds, if not less, to stratify this