perf: Optimize array_positions() for scalar needle

[neilconway]

Which issue does this PR close?

• Closes Optimize 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    │
  └────────────────┴────────┴────────┴─────────┘