Add gbenchmarks for string substrings functions

[davidwendt]

Reference #5698
This creates a gbenchmark for the 4 variations of cudf::strings::slice_strings() API. The benchmarks measures various sized rows as well as strings lengths.
This PR also includes changes to substring.cu implementation cleaning up the code and using the more efficient make_strings_children. This change improved performance for all 4 functions on average by 2-3x.

[codecov]

Codecov Report

Merging #7603 (b60ab0c) into branch-0.19 (7871e7a) will increase coverage by 0.12%.
The diff coverage is 93.22%.

@@               Coverage Diff               @@
##           branch-0.19    #7603      +/-   ##
===============================================
+ Coverage        81.86%   81.99%   +0.12%     
===============================================
  Files              101      101              
  Lines            16884    16989     +105     
===============================================
+ Hits             13822    13930     +108     
+ Misses            3062     3059       -3     

Impacted Files	Coverage Δ
python/cudf/cudf/core/index.py	92.86% <ø> (ø)
python/cudf/cudf/core/column/column.py	87.73% <83.33%> (-0.03%)	⬇️
python/cudf/cudf/core/column/numerical.py	94.85% <85.71%> (-0.17%)	⬇️
python/cudf/cudf/core/frame.py	89.02% <89.47%> (+<0.01%)	⬆️
python/cudf/cudf/core/column/decimal.py	92.75% <90.32%> (-2.12%)	⬇️
python/cudf/cudf/core/dataframe.py	90.45% <95.00%> (-0.01%)	⬇️
python/cudf/cudf/core/series.py	91.24% <95.55%> (+0.46%)	⬆️
python/cudf/cudf/core/column/string.py	86.55% <100.00%> (+0.05%)	⬆️
python/cudf/cudf/core/column_accessor.py	95.28% <100.00%> (-0.03%)	⬇️
python/cudf/cudf/core/dtypes.py	90.27% <100.00%> (+0.54%)	⬆️
... and 9 more

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[kkraus14]

CMake LGTM

[davidwendt]

@gpucibot merge