[ENH] maintain sorted array for conditional join

[samukweku]

This is the third part of a series of PRs that ultimately adds support for aggregations within conditional_join. the numba code now uses an array that is kept sorted, based on grantjenks' sortedcontainers implementation. In one case, perf. improved by about 50x compared to the current implementation; too large a perf diff?

PR Description

Please describe the changes proposed in the pull request:

• maintain a sorted array to get matching indices. this kicks in if there isn't a faster option (monotonic regions)
• fail early if df or right is empty

This PR relates to #1269, #1396 and #1397 .

Please tag maintainers to review.

• @ericmjl

[ericmjl]

🚀 Deployed on https://deploy-preview-1398--pyjanitor.netlify.app

[codecov]

Codecov Report

Attention: Patch coverage is 94.44444% with 14 lines in your changes missing coverage. Please review.

Project coverage is 83.72%. Comparing base (6e77fbc) to head (d3c5772).
Report is 4 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1398      +/-   ##
==========================================
- Coverage   89.07%   83.72%   -5.35%     
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  Files          87       87              
  Lines        5374     5857     +483     
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+ Hits         4787     4904     +117     
- Misses        587      953     +366