perf: Optimize array_positions() for scalar needle

[neilconway]

Which issue does this PR close?

• ClosesOptimize array_positions() for scalar needle #20769.

Rationale for this change

array_positions previously compared the needle against each row's sub-array individually. When the needle is a scalar (the common case), we can do a single bulk arrow_ord::cmp::not_distinct comparison against the entire flat values buffer and then walk the result bitmap, which is significantly faster: the speedup on the array_positions() microbenchmarks ranges from 5x to 40x, depending on the size of the array.

The same pattern has already been applied to array_position (#20532), and previously to other array UDFs.

What changes are included in this PR?

• Add benchmarks for array_positions.
• Implement bulk-comparison optimization
• Refactor array_position's existing fast path slightly for consistency
• Code cleanup to use "haystack" and "needle" consistently, not vague terms like "list_array" and "element"
• Add unit tests for array_positions with sliced ListArrays, for peace of mind
• Add unit tests for sliced lists and sliced lists with nulls for the new array_positions fast path.

Are these changes tested?

Yes.

Are there any user-facing changes?

No.

AI usage

Multiple AI tools were used to iterate on this PR. I have reviewed and understand the resulting code.

[neilconway]

Benchmarks:

 ┌────────────────┬────────┬────────┬─────────┐
│ Benchmark │ Before │ After │ Speedup │
├────────────────┼────────┼────────┼─────────┤
│ found_once/10 │ 1.46ms │ 39µs │ 37x │
├────────────────┼────────┼────────┼─────────┤
│ found_many/10 │ 1.59ms │ 67µs │ 24x │
├────────────────┼────────┼────────┼─────────┤
│ not_found/10 │ 1.28ms │ 32µs │ 40x │
├────────────────┼────────┼────────┼─────────┤
│ found_once/100 │ 1.97ms │ 140µs │ 14x │
├────────────────┼────────┼────────┼─────────┤
│ found_many/100 │ 4.06ms │ 501µs │ 8x │
├────────────────┼────────┼────────┼─────────┤
│ not_found/100 │ 1.82ms │ 132µs │ 14x │
├────────────────┼────────┼────────┼─────────┤
│ found_once/500 │ 4.05ms │ 601µs │ 6.7x │
├────────────────┼────────┼────────┼─────────┤
│ found_many/500 │ 12.0ms │ 2.51ms │ 4.8x │
├────────────────┼────────┼────────┼─────────┤
│ not_found/500 │ 3.82ms │ 588µs │ 6.5x │
└────────────────┴────────┴────────┴─────────┘

[neilconway]

FYI @comphead -- this PR implements a similar optimization for array_positions() that we previously shipped for array_position (#20532).

[comphead]

Thanks @neilconway numbers look great, I left some minors

[neilconway]

@comphead Thanks for the review! I addressed your comments.

[alamb]

🚀