Complete and fixup split of arrow::array::builder module (#1843)

[tustvold]

Which issue does this PR close?

Closes #1843

Rationale for this change

See below

What changes are included in this PR?

Follow on to #1879 which

• Restores the lost changes from Add vec-inspired APIs to BufferBuilder (#1850) #1860
• Moves some stuff out of decimal_builder that didn't belong

Are there any user-facing changes?

No

[alamb]

Looks good to me -- thanks @tustvold

cc @DaltonModlin

[codecov-commenter]

Codecov Report

Merging #1928 (32cb25a) into master (9cd591d) will increase coverage by 0.01%.
The diff coverage is 58.15%.

❗ Current head 32cb25a differs from pull request most recent head 4225292. Consider uploading reports for the commit 4225292 to get more accurate results

@@            Coverage Diff             @@
##           master    #1928      +/-   ##
==========================================
+ Coverage   83.41%   83.42%   +0.01%     
==========================================
  Files         214      217       +3     
  Lines       57004    56981      -23     
==========================================
- Hits        47550    47538      -12     
+ Misses       9454     9443      -11     

Impacted Files	Coverage Δ
arrow/src/array/builder/decimal_builder.rs	76.81% <0.00%> (+7.62%)	⬆️
arrow/src/array/builder/buffer_builder.rs	85.49% <9.09%> (-7.31%)	⬇️
...row/src/array/builder/fixed_size_binary_builder.rs	48.14% <48.14%> (ø)
arrow/src/array/builder/generic_binary_builder.rs	57.14% <57.14%> (ø)
arrow/src/array/array_binary.rs	94.18% <66.66%> (ø)
arrow/src/array/builder/generic_string_builder.rs	80.00% <80.00%> (ø)
arrow/src/array/builder/union_builder.rs	91.66% <87.50%> (+14.67%)	⬆️
arrow/src/array/data.rs	84.16% <100.00%> (ø)
parquet/src/encodings/encoding.rs	93.62% <0.00%> (+0.19%)	⬆️
arrow/src/datatypes/datatype.rs	65.79% <0.00%> (+0.37%)	⬆️

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[tustvold]

We also lost #1842