Optimization for SemanticSplitRange searching

[benbrandt]

Leverages the fact that these ranges are guaranteed to be sorted now. Rather than doing a retain, which likely moves items in the vec and also has to iterate over all values, instead there is a cursor to extract the desired slice, and we can move the cursor to the first item that would still be in the range we desire.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.65%. Comparing base (ac9f17a) to head (d99387e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #219   +/-   ##
=======================================
  Coverage   99.65%   99.65%           
=======================================
  Files          11       11           
  Lines        2036     2047   +11     
=======================================
+ Hits         2029     2040   +11     
  Misses          7        7           

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