Declare the value_length of decimal array as a const

[HaoYang670]

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

Which issue does this PR close?

Closes #1967.

Rationale for this change

The value length of Decimal128 is always 16 bytes, so declare it as a constant.

Are there any user-facing changes?

Yes.

[codecov-commenter]

Codecov Report

Merging #1968 (d9d7439) into master (1a5dcb7) will increase coverage by 0.00%.
The diff coverage is 75.00%.

@@           Coverage Diff           @@
##           master    #1968   +/-   ##
=======================================
  Coverage   83.51%   83.52%           
=======================================
  Files         221      221           
  Lines       57222    57220    -2     
=======================================
  Hits        47791    47791           
+ Misses       9431     9429    -2     

Impacted Files	Coverage Δ
arrow/src/array/array_binary.rs	94.04% <75.00%> (-0.15%)	⬇️
parquet/src/encodings/encoding.rs	93.62% <0.00%> (+0.19%)	⬆️
parquet_derive/src/parquet_field.rs	65.98% <0.00%> (+0.22%)	⬆️
...row/src/array/builder/string_dictionary_builder.rs	91.72% <0.00%> (+0.75%)	⬆️

---

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 1a5dcb7...d9d7439. Read the comment docs.

[tustvold]

This makes sense to me, but perhaps someone who s more familiar with the semantics of DecimalArray can weigh in here

[HaoYang670]

Thank you for your review @tustvold .

[HaoYang670]

@viirya Could you please have a look?

[viirya]

Looks okay to me.