perf: do not recompute the intersection when backtracking

[Eh2406]

This eliminates the intersection calls made when backtracking. Unfortunately, it is within the noise on all of my standard benchmarks. However it does cause dramatic speedups for #135 (comment) (I really need to make a ron file of this, so that I can add it to my normal benchmarking suite.)

Based on the flame graphs we reviewed in Office Hours today I suspect this will make a solid difference for @konstin workloads. It also unlocks more impactful simplifications in satisfier search.

[mpizenberg]

Let me know if my understanding is correct.

This PR is a memory/compute trade off.

Instead of only keeping the latest assignments intersection, we keep a record of intersections at each stage by storing it in dated_derivations.

This change prevents the need to recompute intersections when backtracking since dated derivations also store the intersection at the corresponding time.

I suppose AssignmentsIntersection::Derivations could now use a reference "&" to the latest dated derivation intersection instead of its own copy?

[konstin]

Great work, that makes the two bad cases i test a good bit faster:

main: f64c499
branch: 8d5cfbb

tf-models-nightly main
real    0m5,509s
user    0m5,228s
sys     0m0,274s

tf-models-nightly branch
real    0m2,905s
user    0m2,630s
sys     0m0,272s

bio_embeddings main
real    2m7,274s
user    2m5,410s
sys     0m1,396s

bio_embeddings branch
real    1m42,388s
user    1m40,671s
sys     0m1,533s

[Eh2406]

@mpizenberg, yes your understanding is correct.

I suppose AssignmentsIntersection::Derivations could now use a reference "&" to the latest dated derivation intersection instead of its own copy?

Theoretically yes. But then PackageAssignments become self-referential. I experimented not storing anything in AssignmentsIntersection::Derivations and retrieving the intersection with pa.dated_derivations.last().unwrap() but the code was too convoluted for me to find the bug I had introduced. We could share the memory with an RC but given its only can be used in two places it's probably the same cost as a clone. So I decided any larger change should be done in a follow-up.

[Eh2406]

I fixed the comment and improved some redundancy. This should be ready for a final review.

I also generated a Ron file inspired by the example I've been using for testing. slow_135_0_u16_NumberVersion.txt (change from txt to ron to use). It's runtime gets about 50% faster after this PR. How realistic is it 🤷 .