perf: Compute array_length of first dimension from offsets

[neilconway]

Which issue does this PR close?

• Closes Compute array_length from offsets #25119.

Rationale for this change

array_length worked by forming a slice of values for each logical row in the given dimension, and then appending the length of that slice to the result. That is inefficient; when computing the length of the first array dimension, we can get that from the offset buffer instead, which is ~200x faster for typical inputs. For other array dimensions, we fallback to the previous code path.

Benchmarks: (M4 Max)

• list/8192, 175.379 µs -> 1.137 µs, -99.35%
• fixed_size_list/8192, 174.770 µs -> 0.963 µs, -99.45%
• list/1, 0.171 µs -> 0.088 µs, -48.48%

What changes are included in this PR?

• Compute array_length of dimension 1 for List and LargeList from offsets
• Compute array_length of dimension 1 for FixedSizeList from the FSL's size
• Treat array_length(x, 1) as equivalent to array_length(x)
• Simplify the code to remove a no-longer-necessary macro
• Add unit tests
• Add benchmark for array_length

What is the testing strategy for this PR?

Existing tests pass; new unit tests added.

Are there any user-facing changes?

No.

[AdamGS]

awesome improvement, LGTM!

[neilconway]

@AdamGS @Rich-T-kid Thanks for the reviews!

[codecov-commenter]

Codecov Report

❌ Patch coverage is 84.31373% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.80%. Comparing base (4048898) to head (8d918c3).

Files with missing lines	Patch %	Lines
datafusion/functions-nested/src/length.rs	84.31%	8 Missing and 8 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25121      +/-   ##
==========================================
- Coverage   81.80%   81.80%   -0.01%     
==========================================
  Files        1130     1130              
  Lines      417754   417842      +88     
  Branches   417754   417842      +88     
==========================================
+ Hits       341754   341823      +69     
- Misses      55875    55883       +8     
- Partials    20125    20136      +11     

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