Mark boolean comparison kernels public

[alamb]

Which issue does this PR close?

Closes #912

Rationale for this change

The boolean kernels added in #844 by @Dandandan are not marked pub and thus can not be used. For example:

https://docs.rs/arrow/6.1.0/arrow/compute/kernels/comparison/index.html?search=eq_bool

What changes are included in this PR?

mark the boolean kernels public

Are there any user-facing changes?

kernels are usable

[Dandandan]

Oops 😬 ! Thank you @alamb

[codecov-commenter]

Codecov Report

Merging #913 (1f50200) into master (d5a4bd7) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 1f50200 differs from pull request most recent head 1ca308a. Consider uploading reports for the commit 1ca308a to get more accurate results

@@           Coverage Diff           @@
##           master     #913   +/-   ##
=======================================
  Coverage   82.29%   82.30%           
=======================================
  Files         168      168           
  Lines       48028    48028           
=======================================
+ Hits        39526    39530    +4     
+ Misses       8502     8498    -4     

Impacted Files	Coverage Δ
arrow/src/compute/kernels/comparison.rs	92.63% <100.00%> (ø)
parquet/src/encodings/encoding.rs	93.71% <0.00%> (+0.19%)	⬆️
parquet_derive/src/parquet_field.rs	66.21% <0.00%> (+0.22%)	⬆️
arrow/src/array/transform/mod.rs	85.47% <0.00%> (+0.27%)	⬆️

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[alamb]

Oops 😬 ! Thank you @alamb

LOL I should have caught it in review