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

Merging #1879 (c7c6cb7) into master (2e3d8dd) will decrease coverage by 0.01%.
The diff coverage is 86.97%.

@@            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     

Impacted Files	Coverage Δ
arrow/src/array/mod.rs	100.00% <ø> (ø)
arrow/src/array/builder/decimal_builder.rs	69.18% <69.18%> (ø)
arrow/src/array/builder/boolean_builder.rs	75.75% <75.75%> (ø)
arrow/src/array/builder/union_builder.rs	76.99% <76.99%> (ø)
arrow/src/array/builder/struct_builder.rs	77.47% <77.47%> (ø)
.../src/array/builder/primitive_dictionary_builder.rs	78.18% <78.18%> (ø)
arrow/src/array/builder/map_builder.rs	81.63% <81.63%> (ø)
arrow/src/array/builder/fixed_size_list_builder.rs	87.61% <87.61%> (ø)
arrow/src/array/builder/primitive_builder.rs	91.88% <91.88%> (ø)
...row/src/array/builder/string_dictionary_builder.rs	92.23% <92.23%> (ø)
... and 5 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2e3d8dd...c7c6cb7. Read the comment docs.

[alamb]

Thanks again @DaltonModlin