Improve performance of variable resolver

[bnoordhuis]

Switch to a hash table when the number of variables grows beyond a threshold.

Speeds up the test case from the linked issue by about 70%.

Fixes: #456

[bnoordhuis]

I'm going to tweak it a little more before I land it, stay tuned.

[bnoordhuis]

Switching the hash function resulted in a whopping 15% fewer hash collisions. Pretty big deal.

Catastrophic collisions are still going to be an issue once the table grows to a few million elements but I hope and assume that's never going to happen in the real world. Even that monster from the linked issue has < 20,000 elements.

[saghul]

Nice! Random question: we seem to have a few htables around now, do you see an opportunity to DRY some out? Not in this PR of course.

[richarddd]

Nice! Random question: we seem to have a few htables around now, do you see an opportunity to DRY some out? Not in this PR of course.

This would be great. Maybe even pulling in something external? https://github.com/JacksonAllan/Verstable :D

[bnoordhuis]

we seem to have a few htables around now, do you see an opportunity to DRY some out?

Dunno. They're all just different enough from each other that it's not a slam dunk DRY job.

I did notice that, compared to perl hash, shape_hash and get_index_hash have like 5x the number of hash collisions on the (limited) inputs I tested it with.

Swapping the hash function didn't make a measurable difference in my quick testing though, likely because the shape and IC hash tables just don't get that big.