`Relation::from_antijoin` is broken

[ecstatic-morse]

Relation::from_antijoin always returns an empty Relation, regardless of its inputs. That's because the antijoin helper, which takes a JoinInput as its first parameter, operates only on recent tuples.

datafrog/src/join.rs

Lines 65 to 73 in 5bda2f0

	let results = input1
	.recent()
	.iter()
	.filter(|(ref key, _)| {
	tuples2 = gallop(tuples2, |k| k < key);
	tuples2.first() != Some(key)
	})
	.map(|(ref key, ref val)| logic(key, val))
	.collect::<Vec<_>>();

This is correct for variables, but Relations, which don't change during iteration, only have stable tuples. See #36 (comment) for the reason this must be the case.

To fix this, we should refactor the antijoin helper to work directly on Relations, and pass the proper input from Variable::from_antijoin and Relation::from_antijoin. A regression test is needed as well.

[ecstatic-morse]

Fixed in #42.