Restricting recursiveCallsAndSurroundingUnfoldings to look for recursive calls only

[marcoeilers]

@JonasAlaif found that predicate triggers don't apply just to recursive calls but to any function calls, which is almost certainly not intended, contradicts the name of the function that looks for relevant calls, and can easily lead to matching loops (see #895).

While there are other issues with the way the triggers are generated, this PR just fixes the most glaring issue and restricts the triggers to look for actual recursive calls only.

[JonasAlaif]

I'm a bit worried about solving it this way wrt to mutually recursive calls however

[marcoeilers]

We could of course look at the actual function call graph to also support mutual recursion. I didn't want to do that because I've seen call graph computation being slow, and I'm not sure I've ever seen a case where this would matter. But actually call graph computation was slow because we're hashing functions for no reason (see #867), so I'll just fix that, and then we can use the call graph here.