Allow the output of a join to be filtered by an arbitrary predicate

[ecstatic-morse]

The purpose is to eliminate reflexive subset relations in Polonius. Currently, we do this by manually filtering the subset relation at each stage, but this is inefficient and makes things a bit harder to reason about. This change allows us to write the equivalent of the following rule.

subset(o1, o3, p) :-
    subset(o1, o2, p),
    subset(o2, o3, p),
    o1 != o3.

The signature for from_join_filtered is maximally expressive, allowing the user to inspect the shared key and both values when deciding whether to filter. I think this is good, but it's possible there's a more restrictive API that would achieve the same thing?

[lqd]

This is exactly what I had in mind when I added the hacky symmetry filtering to polonius: a way to filter_map the emitted tuples for these cases. So this looks good to me as well, and I personally don't mind the maximally expressive vs restricted API (but that's an interesting question indeed).

[lqd]

thanks a bunch!