Add Decimal to CsvWriter and improve debug display

[alippai]

Which issue does this PR close?

Closes #405.

Rationale for this change

A simple CSV serializer for Decimals and matching debug format improvement.

What changes are included in this PR?

Decimal -> string conversion.

Are there any user-facing changes?

The debug display of DecimalArray(precision:10, scale:2) is changed from:

12345

to

123.45

Two things to consider for any reviewer:

1. I might be wrong with try_into() and as i32 and other cast. I'm new to Rust and I don't know the exact implications of my introduced changes.
2. For integer-like decimals I still write 1.0 instead of 1. This was a subjective decision, I don't have strong feelings about it. Padding was/is missing as well.

[codecov-commenter]

Codecov Report

Merging #406 (83f5f6b) into master (dc5507a) will increase coverage by 0.00%.
The diff coverage is 89.28%.

@@           Coverage Diff           @@
##           master     #406   +/-   ##
=======================================
  Coverage   82.65%   82.65%           
=======================================
  Files         164      164           
  Lines       45458    45478   +20     
=======================================
+ Hits        37573    37592   +19     
- Misses       7885     7886    +1     

Impacted Files	Coverage Δ
arrow/src/util/display.rs	32.46% <60.00%> (+2.73%)	⬆️
arrow/src/array/array_binary.rs	90.13% <92.85%> (-0.03%)	⬇️
arrow/src/csv/writer.rs	83.15% <100.00%> (+0.50%)	⬆️

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[alamb]

I think the code is looking good @alippai -- thank you. I had some suggestions / questions on the API -- @nevi-me / @Dandandan do you have any thoughts on the API?

[alamb]

this looks like a significant improvement to me 👍

[alippai]

@alamb Created DecimalArray::value_as_string() instead of the display.rs implementation.

What I couldn't find is the usage of #[inline] vs #[inline(always)] also pub fn value() is not inlined for DecimalArray but it is for primitive arrays. Is there a rule of thumb for this?

[alippai]

Thanks @nevi-me

[alamb]

Thanks @alippai -- this looks great 👍