refactor: fix confusing impl of indexed_fragments

[wjones127]

The reference to total_fragment_bitmap made no sense. This PR eliminates that, and also fixes very minor inefficiencies.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 78.39%. Comparing base (536e73d) to head (5991d35).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
rust/lance/src/index.rs	86.66%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3047      +/-   ##
==========================================
+ Coverage   78.38%   78.39%   +0.01%     
==========================================
  Files         240      240              
  Lines       77122    78592    +1470     
  Branches    77122    78592    +1470     
==========================================
+ Hits        60449    61614    +1165     
- Misses      13565    13862     +297     
- Partials     3108     3116       +8     

Flag	Coverage Δ
unittests	78.39% <86.66%> (+0.01%)	⬆️

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