Add gbenchmark for strings split/split_record functions

[davidwendt]

Reference #5698
This creates a gbenchmark for the cudf::strings::split and cudf::strings::split_record functions.

This PR also includes changes to split.cu to use device_uvector for temporary buffers instead of device_vector and other minor code cleanup like adding more const decls.

[kkraus14]

cmake lgtm

[codecov]

Codecov Report

Merging #7427 (7551d90) into branch-0.19 (43b44e1) will increase coverage by 0.43%.
The diff coverage is 96.66%.

@@               Coverage Diff               @@
##           branch-0.19    #7427      +/-   ##
===============================================
+ Coverage        81.80%   82.24%   +0.43%     
===============================================
  Files              101      101              
  Lines            16695    17069     +374     
===============================================
+ Hits             13658    14039     +381     
+ Misses            3037     3030       -7     

Impacted Files	Coverage Δ
python/cudf/cudf/core/frame.py	89.35% <ø> (+0.09%)	⬆️
python/cudf/cudf/core/column_accessor.py	95.47% <95.65%> (+2.53%)	⬆️
python/cudf/cudf/core/dataframe.py	90.58% <100.00%> (+0.12%)	⬆️
python/cudf/cudf/utils/gpu_utils.py	53.65% <0.00%> (-4.88%)	⬇️
python/cudf/cudf/core/abc.py	87.23% <0.00%> (-1.14%)	⬇️
python/cudf/cudf/io/feather.py	100.00% <0.00%> (ø)
python/cudf/cudf/comm/serialize.py	0.00% <0.00%> (ø)
python/cudf/cudf/_fuzz_testing/io.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/column/struct.py	100.00% <0.00%> (ø)
python/dask_cudf/dask_cudf/_version.py	0.00% <0.00%> (ø)
... and 43 more

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[trxcllnt]

👍 for device_uvector

[davidwendt]

@gpucibot merge