Add gbenchmark for strings::concatenate

[davidwendt]

Reference #5698
This creates a gbenchmark for cudf::strings::concatenate function. The benchmarks measures various sized rows as well as strings lengths. This PR also includes some changes to combine.cu for cleaning up the code and replacing device_vector usages with device_uvector.

[codecov]

Codecov Report

Merging #7560 (597cbd9) into branch-0.19 (7871e7a) will increase coverage by 0.58%.
The diff coverage is 93.16%.

@@               Coverage Diff               @@
##           branch-0.19    #7560      +/-   ##
===============================================
+ Coverage        81.86%   82.44%   +0.58%     
===============================================
  Files              101      101              
  Lines            16884    17369     +485     
===============================================
+ Hits             13822    14320     +498     
+ Misses            3062     3049      -13     

Impacted Files	Coverage Δ
python/cudf/cudf/core/index.py	93.34% <ø> (+0.48%)	⬆️
python/cudf/cudf/core/series.py	91.65% <ø> (+0.86%)	⬆️
python/cudf/cudf/core/tools/datetimes.py	84.53% <ø> (+0.08%)	⬆️
python/cudf/cudf/utils/cudautils.py	52.94% <ø> (+2.55%)	⬆️
python/cudf/cudf/utils/dtypes.py	89.88% <ø> (+0.37%)	⬆️
python/dask_cudf/dask_cudf/io/orc.py	91.04% <ø> (+0.13%)	⬆️
python/cudf/cudf/core/column/numerical.py	94.83% <87.50%> (-0.20%)	⬇️
python/cudf/cudf/core/frame.py	89.09% <89.47%> (+0.08%)	⬆️
python/cudf/cudf/core/column/column.py	87.86% <90.00%> (+0.10%)	⬆️
python/cudf/cudf/core/column/decimal.py	92.75% <90.32%> (-2.12%)	⬇️
... and 61 more

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[ttnghia]

Why the API is concatenate but the file names are combine.hpp/cu? I would prefer consistency between the function and file names.

[davidwendt]

Why the API is concatenate but the file names are combine.hpp/cu? I would prefer consistency between the function and file names.

The combine.hpp includes cudf::strings::concatenate and cudf::strings::join API declarations.

[nvdbaranec]

One question about set_element_async usage.

[kkraus14]

CMake lgtm

[davidwendt]

@gpucibot merge