Chained hash pipelining in array hashing

[adienes]

the proposed switch in #57509 from 3h - hash_finalizer(x) to hash_finalizer(3h -x) should increase the hash quality of chained hashes, as the expanded expression goes from something like sum((-3)^k * hash(x) for k in ...) to a non-simplifiable composition

this does have the unfortunate impact of long chains of hashes getting a bit slower as there is more data dependency and the CPU cannot work on the next element's hash before combining the previous one (I think --- I'm not particularly an expert on this low level stuff). As far as I know this only really impacts AbstractArray

so, I've implemented a proposal that does some unrolling / pipelining manually to recover AbstractArray hashing performance. in fact, it's quite a lot faster now for most lengths. I tuned the thresholds (8 accumulators, certain length breakpoints) by hand on my own machine.

[oscardssmith]

show performance benchmarks and then lgtm.

[adienes]

#57509 (comment) the vec column contains timing; the data for this PR is under :commit == "pipeline" sorry I should have been more clear in that comment

graphically:

note that this cannot merge before #57509, which is also waiting on #58053