Add Decimal128Iter and Decimal256Iter and do maximum precision/scale check

[viirya]

Which issue does this PR close?

Closes #2139.
Closes #2138.

Rationale for this change

What changes are included in this PR?

Are there any user-facing changes?

[codecov-commenter]

Codecov Report

Merging #2140 (fda770f) into master (19a7189) will decrease coverage by 0.04%.
The diff coverage is 74.19%.

@@            Coverage Diff             @@
##           master    #2140      +/-   ##
==========================================
- Coverage   82.86%   82.82%   -0.05%     
==========================================
  Files         237      237              
  Lines       61296    61425     +129     
==========================================
+ Hits        50792    50874      +82     
- Misses      10504    10551      +47     

Impacted Files	Coverage Δ
arrow/src/array/array_binary.rs	92.87% <ø> (ø)
arrow/src/array/iterator.rs	86.45% <ø> (-13.55%)	⬇️
arrow/src/datatypes/datatype.rs	62.69% <ø> (ø)
arrow/src/compute/kernels/cast.rs	95.79% <50.00%> (ø)
arrow/src/util/decimal.rs	86.92% <68.00%> (-4.59%)	⬇️
arrow/src/array/array_decimal.rs	90.32% <80.00%> (-0.72%)	⬇️
parquet/src/column/reader/decoder.rs	50.00% <0.00%> (-11.34%)	⬇️
...row/src/array/builder/string_dictionary_builder.rs	90.64% <0.00%> (-0.72%)	⬇️
parquet_derive/src/parquet_field.rs	65.98% <0.00%> (+0.22%)	⬆️
... and 1 more

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[tustvold]

Worth adding the above comment as a deprecation hint?

[ursabot]

Benchmark runs are scheduled for baseline = 1621c71 and contender = 73153fe. 73153fe is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ec2-t3-xlarge-us-east-2] ec2-t3-xlarge-us-east-2
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on test-mac-arm] test-mac-arm
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ursa-i9-9960x] ursa-i9-9960x
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ursa-thinkcentre-m75q] ursa-thinkcentre-m75q
Buildkite builds:
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java