Add gbenchmarks for strings filter functions

[davidwendt]

Reference #5698
This creates a gbenchmark for the cudf::strings::filter_characters, cudf::strings::filter_characters_of_type, and cudf::strings::strip functions.

This PR also includes changes to strip.cu and filter_chars to use the more efficient make_strings_children utility. This improved performance on these functions by 2x on average.

[kkraus14]

cmake lgtm

[codecov]

Codecov Report

Merging #7438 (a44098e) into branch-0.19 (43b44e1) will increase coverage by 0.03%.
The diff coverage is 96.66%.

@@               Coverage Diff               @@
##           branch-0.19    #7438      +/-   ##
===============================================
+ Coverage        81.80%   81.84%   +0.03%     
===============================================
  Files              101      101              
  Lines            16695    16707      +12     
===============================================
+ Hits             13658    13674      +16     
+ Misses            3037     3033       -4     

Impacted Files	Coverage Δ
python/cudf/cudf/core/frame.py	89.25% <ø> (ø)
python/cudf/cudf/core/column_accessor.py	95.31% <95.65%> (+2.37%)	⬆️
python/cudf/cudf/core/dataframe.py	90.46% <100.00%> (+<0.01%)	⬆️

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[codereport]

lgtm, other than a couple small things

[codereport]

👍

[kkraus14]

@gpucibot merge