Fix slicing and arrow representations of decimal columns

[vyasr]

Slices of decimal columns were not passing the appropriate offset to the new column views being created. Additionally, the conversion of decimal columns to and from arrow did not include the offset. This made all slices of decimal columns appear invalid.

Thanks to @shwina for helping me trace these bugs back to their source!

Edit
This PR also now includes an additional fix for the behavior of slices of decimal columns that should generate empty data sets (e.g. starting a slice past the end of a column).

[codecov]

Codecov Report

Merging #7755 (d58c8f8) into branch-0.19 (7871e7a) will increase coverage by 0.80%.
The diff coverage is n/a.

❗ Current head d58c8f8 differs from pull request most recent head fae2063. Consider uploading reports for the commit fae2063 to get more accurate results

@@               Coverage Diff               @@
##           branch-0.19    #7755      +/-   ##
===============================================
+ Coverage        81.86%   82.67%   +0.80%     
===============================================
  Files              101      103       +2     
  Lines            16884    17548     +664     
===============================================
+ Hits             13822    14507     +685     
+ Misses            3062     3041      -21     

Impacted Files	Coverage Δ
python/cudf/cudf/utils/dtypes.py	83.54% <0.00%> (-5.97%)	⬇️
python/cudf/cudf/utils/gpu_utils.py	53.65% <0.00%> (-4.88%)	⬇️
python/cudf/cudf/core/column/lists.py	87.32% <0.00%> (-4.08%)	⬇️
python/dask_cudf/dask_cudf/backends.py	87.50% <0.00%> (-2.13%)	⬇️
python/cudf/cudf/core/abc.py	87.23% <0.00%> (-1.14%)	⬇️
python/cudf/cudf/core/column/decimal.py	94.36% <0.00%> (-0.51%)	⬇️
python/cudf/cudf/core/column/column.py	87.53% <0.00%> (-0.23%)	⬇️
python/cudf/cudf/utils/utils.py	85.36% <0.00%> (-0.07%)	⬇️
python/cudf/cudf/io/feather.py	100.00% <0.00%> (ø)
python/cudf/cudf/utils/ioutils.py	78.71% <0.00%> (ø)
... and 49 more

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[brandon-b-miller]

Looks good - there's some tests for offset and size in test_column.py, I think this was not seen earlier because that set of tests only covers the dtypes that pandas supports. I'd suggest adding a small test there.

[brandon-b-miller]

Couple quick test cases and this should be good to go.

[kkraus14]

@gpucibot merge