feat: memory efficient and performant gini

[sjsrey]

Motivation for this and a performance evaluation are here. This replaces #170

[codecov]

Codecov Report

Merging #195 (a48284c) into master (08de8f8) will decrease coverage by 0.17%.
The diff coverage is 26.92%.

@@            Coverage Diff             @@
##           master     #195      +/-   ##
==========================================
- Coverage   81.53%   81.35%   -0.18%     
==========================================
  Files         117      117              
  Lines        3396     3417      +21     
==========================================
+ Hits         2769     2780      +11     
- Misses        627      637      +10     

Impacted Files	Coverage Δ
segregation/singlegroup/gini.py	55.55% <26.92%> (-40.28%)	⬇️
segregation/_version.py	39.16% <0.00%> (+2.67%)	⬆️

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[knaaptime]

sweet

[renanxcortes]

Cool!