perf: more efficient intersection

[Eh2406]

I got nerds not by astral-sh#6. Which points out that intersection is slower than it needs to be when the basic operations on version are expensive. Here's what I've come up with.

The ideas are:

1. Like that PR, avoid cloning till the end
2. Instead of using next_if and eq keep track of which iter to advance directly.
3. Because we know which iter end came from, and we know that for each iterator start < end, check for validity before determining start
4. Don't redundantly check e > i

branch	clone to reject	compares to reject	clones to accept	comparisons to accept
dev	2	5	2	5
zanieb/#6	0	5	2	5
this	0	2	2	3

I tested this synthetically by making a version that increasing a global counter every time eq/cmp/clone/drop is called. This global counter goes up much (~30%) less after this PR.
Even on our normal benchmarks, where these operations are very cheap (a handful of CPU cycles), we see improvements (although only a few percent).

cc @konstin for real-world impact.
cc @baszalmstra mamba-org/resolvo#2 (comment) I don't know if these are improvements you'd be interested in include in your copy of this type.

[konstin]

I tried this branch on a pathological test case and it reduced the time from 10s to 3.6s!

[Eh2406]

That is wonderful to hear! As soon as this gets a review, let's get this merged!

[zanieb]

This looks reasonable to me, but may benefit from a bit of documentation?

[Eh2406]

I rarely provide enough documentation or use good names for things. Always feel comfortable asking me to improve! Anything in particular you want me to clarify?

[zanieb]

I think basically anywhere you're doing something unintuitive for performance reasons as outlined in your bullets in the pull request summary you'd benefit from adding a comment explaining what's going on e.g.

// We can conclude e > i and do not check again for perf

[Eh2406]

Here's a small dissertation. Let me know if any of it made sense.

[Eh2406]

If people think of improvements to the documentation I added, PR's are welcome. So I'm going to merge.