Split up arrow::array::builder module (#1843)

[DaltonModlin]

Which issue does this PR close?

Closes #1843

Rationale for this change

Currently, arrow::array::builder contains 12 distinct builders; breaking the builders into submodules makes test coverage and functional differences more obvious.

Are there any user-facing changes?

No

[tustvold]

Thank you for this 👍, I plan to go through this this evening unless someone else gets there first. Left some minor comments from a cursory run through.

Edit: CI appears to be unhappy

[alamb]

Thank you very much @DaltonModlin

this PR appears to have a conflict -- could you possible resolve it and I will then merge it in?

Thanks!

[codecov-commenter]

Codecov Report

❌ Patch coverage is 86.97648% with 227 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.47%. Comparing base (2e3d8dd) to head (c7c6cb7).

Files with missing lines	Patch %	Lines
arrow/src/array/builder/decimal_builder.rs	69.18%	49 Missing ⚠️
arrow/src/array/builder/struct_builder.rs	77.47%	41 Missing ⚠️
arrow/src/array/builder/union_builder.rs	76.99%	26 Missing ⚠️
arrow/src/array/builder/primitive_builder.rs	91.88%	19 Missing ⚠️
arrow/src/array/builder/map_builder.rs	81.63%	18 Missing ⚠️
arrow/src/array/builder/boolean_builder.rs	75.75%	16 Missing ⚠️
arrow/src/array/builder/fixed_size_list_builder.rs	87.61%	13 Missing ⚠️
.../src/array/builder/primitive_dictionary_builder.rs	78.18%	12 Missing ⚠️
arrow/src/array/builder/generic_list_builder.rs	96.65%	10 Missing ⚠️
arrow/src/array/builder/buffer_builder.rs	92.80%	9 Missing ⚠️
... and 2 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1879      +/-   ##
==========================================
- Coverage   83.48%   83.47%   -0.02%     
==========================================
  Files         203      214      +11     
  Lines       57141    57163      +22     
==========================================
+ Hits        47705    47717      +12     
- Misses       9436     9446      +10     

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[alamb]

Thanks again @DaltonModlin