Miscellaneous column cleanup

[vyasr]

This PR contains an array of minor improvements to column classes to improve readability and simplify some implementations.

[codecov]

Codecov Report

Merging #9370 (8de897f) into branch-21.12 (ab4bfaa) will decrease coverage by 0.11%.
The diff coverage is n/a.

@@               Coverage Diff                @@
##           branch-21.12    #9370      +/-   ##
================================================
- Coverage         10.79%   10.67%   -0.12%     
================================================
  Files               116      117       +1     
  Lines             18869    19705     +836     
================================================
+ Hits               2036     2104      +68     
- Misses            16833    17601     +768     

Impacted Files	Coverage Δ
python/dask_cudf/dask_cudf/sorting.py	92.90% <0.00%> (-1.21%)	⬇️
python/cudf/cudf/io/csv.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/hdf.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/orc.py	0.00% <0.00%> (ø)
python/cudf/cudf/__init__.py	0.00% <0.00%> (ø)
python/cudf/cudf/_version.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/abc.py	0.00% <0.00%> (ø)
python/cudf/cudf/api/types.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/dlpack.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/frame.py	0.00% <0.00%> (ø)
... and 61 more

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[shwina]

Looks great to me! Glad to see all the as_column->build_column changes

[isVoid]

Just some minor comments. Also not familiar with the changes in test_dlpack, can you explain a bit?

[isVoid]

Looks great to me! Glad to see all the as_column->build_column changes

Also glad to see isna -> isnull changes

[vyasr]

Just some minor comments. Also not familiar with the changes in test_dlpack, can you explain a bit?

I just removed ColumnBase.to_dlpack entirely, so the corresponding tests had to be removed as well. We never need to convert a column into a dlpack tensor, only higher-level objects (Frames), so there's no reason to support this.

[shwina]

@gpucibot merge