Move DecimalArray to array_decimal.rs

[HaoYang670]

Signed-off-by: remzi 13716567376yh@gmail.com

Which issue does this PR close?

Closes #1985.

Rationale for this change

1. DecimalArray is a top level logical type: https://github.com/apache/arrow/blob/master/format/Schema.fbs#L405
2. DecimalArray has no relationship with binary arrays: Personally, I don't think the decimal array is a logical type of FixedSizeBinaryArray because the value_length is a constant (128bits or 256bits) but not user-defined.
3. If we add Decimal256Array support in the future, putting the code in array_decimal.rs could be more readable.

What changes are included in this PR?

Are there any user-facing changes?

Yes.

[codecov-commenter]

Codecov Report

Merging #1986 (6595119) into master (4350b8c) will increase coverage by 0.00%.
The diff coverage is 93.67%.

@@           Coverage Diff           @@
##           master    #1986   +/-   ##
=======================================
  Coverage   83.57%   83.58%           
=======================================
  Files         221      222    +1     
  Lines       57495    57495           
=======================================
+ Hits        48054    48055    +1     
+ Misses       9441     9440    -1     

Impacted Files	Coverage Δ
arrow/src/array/array_binary.rs	94.20% <ø> (+0.16%)	⬆️
arrow/src/array/mod.rs	100.00% <ø> (ø)
arrow/src/array/array_decimal.rs	93.67% <93.67%> (ø)
parquet_derive/src/parquet_field.rs	65.75% <0.00%> (-0.23%)	⬇️
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]

LGTM! 👍

I don't think this is an API change as the module hierarchy isn't public

[HaoYang670]

Thank you for your review @tustvold @liukun4515!

[liukun4515]

LGTM! 👍

I don't think this is an API change as the module hierarchy isn't public

Sorry for this error label.
I did't review the mode file carefully for the decimal about pub use self::array_decimal::DecimalArray;