ARROW-4885: [C++/Python] Enable Decimal parsing in CSV

[emkornfield]

• Create a new Decimal128 converter in csv (copies some code for eliminating white space, please let me know if this should be factored out).
• Add python unit test
• I filed ARROW-5699 to track 2 performance enhancements:
  • Add an UnsafeAppend and use it on the Decimal128Builder
  • Avoid multiple string copies in Decimal128::FromString.

[codecov-io]

Codecov Report

Merging #4660 into master will increase coverage by 0.56%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4660      +/-   ##
==========================================
+ Coverage   88.38%   88.95%   +0.56%     
==========================================
  Files         904      713     -191     
  Lines      112252    97095   -15157     
  Branches     1420        0    -1420     
==========================================
- Hits        99216    86372   -12844     
+ Misses      12674    10723    -1951     
+ Partials      362        0     -362

Impacted Files	Coverage Δ
python/pyarrow/tests/test_csv.py	98.34% <100%> (ø)	⬆️
cpp/src/arrow/csv/converter-test.cc	100% <100%> (ø)	⬆️
cpp/src/arrow/csv/converter.cc	96.72% <100%> (-1.02%)	⬇️
go/arrow/ipc/writer.go
go/arrow/math/uint64_amd64.go
go/arrow/memory/memory_avx2_amd64.go
go/arrow/ipc/file_reader.go
js/src/builder/index.ts
js/src/enum.ts
... and 187 more

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[pitrou]

Thanks for doing this. Just two minor points.

[pitrou]

+1, will merge when green. Thanks @emkornfield !