cdef packed_columns as cppclass instead of struct

[charlesbluca]

Closes #8935

Defines packed_columns in Cython as a cppclass instead of a struct, so that it is actually destroyed when a containing PackedColumns object is no longer referenced; see cython/cython#3226.

[charlesbluca]

@gpucibot merge

[codecov]

Codecov Report

Merging #8936 (7b57c43) into branch-21.10 (18f7c01) will decrease coverage by 0.06%.
The diff coverage is n/a.

❗ Current head 7b57c43 differs from pull request most recent head 79655f1. Consider uploading reports for the commit 79655f1 to get more accurate results

@@               Coverage Diff                @@
##           branch-21.10    #8936      +/-   ##
================================================
- Coverage         10.67%   10.61%   -0.07%     
================================================
  Files               110      116       +6     
  Lines             18271    18996     +725     
================================================
+ Hits               1951     2017      +66     
- Misses            16320    16979     +659     

Impacted Files	Coverage Δ
python/cudf/cudf/__init__.py	0.00% <ø> (ø)
python/cudf/cudf/core/__init__.py	0.00% <ø> (ø)
python/cudf/cudf/core/column/categorical.py	0.00% <ø> (ø)
python/cudf/cudf/core/column/column.py	0.00% <ø> (ø)
python/cudf/cudf/core/column/lists.py	0.00% <ø> (ø)
python/cudf/cudf/core/column/methods.py	0.00% <ø> (ø)
python/cudf/cudf/core/column/numerical.py	0.00% <ø> (ø)
python/cudf/cudf/core/column/string.py	0.00% <ø> (ø)
python/cudf/cudf/core/column/struct.py	0.00% <ø> (ø)
python/cudf/cudf/core/dataframe.py	0.00% <ø> (ø)
... and 80 more

---

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 7f704d6...79655f1. Read the comment docs.