Add length kernel support for List Array

[HaoYang670]

Which issue does this PR close?

Closes #1470 .

What changes are included in this PR?

1. add length kernel support for list array
2. do some clean up work, including: rename private functions (make names more consistent), format the generic type bounds and update the doc.

Are there any user-facing changes?

Doc of the public function length is updated.

[codecov-commenter]

Codecov Report

Merging #1488 (aa6a138) into master (2932da3) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1488      +/-   ##
==========================================
+ Coverage   82.71%   82.73%   +0.02%     
==========================================
  Files         187      188       +1     
  Lines       54255    54319      +64     
==========================================
+ Hits        44877    44941      +64     
  Misses       9378     9378              

Impacted Files	Coverage Δ
arrow/src/compute/kernels/length.rs	100.00% <100.00%> (ø)
parquet_derive/src/parquet_field.rs	65.75% <0.00%> (-0.23%)	⬇️
parquet/src/encodings/encoding.rs	93.37% <0.00%> (-0.19%)	⬇️
parquet/src/arrow/array_reader/builder.rs	66.30% <0.00%> (ø)
parquet/src/arrow/arrow_writer.rs	97.65% <0.00%> (+0.09%)	⬆️
parquet/src/arrow/levels.rs	84.80% <0.00%> (+0.12%)	⬆️
parquet/src/arrow/array_reader.rs	84.36% <0.00%> (+5.63%)	⬆️

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[viirya]

Suggested change

	/// * length of null is null.
	/// * length is in number of bytes
	/// * length of null is null.

[HaoYang670]

Done!

[viirya]

Looks good. Thanks.

[alamb]

Thanks @HaoYang670 and @viirya