Speed up index_chains

[grst]

Closes #386

• CHANGELOG.md updated
• Tests added (For bug fixes or new features)
• Tutorial updated (if necessary)

[codecov]

Codecov Report

Attention: 10 lines in your changes are missing coverage. Please review.

Comparison is base (8cc7e73) 81.17% compared to head (3bd4b1b) 80.17%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #444      +/-   ##
==========================================
- Coverage   81.17%   80.17%   -1.01%     
==========================================
  Files          49       49              
  Lines        3868     3924      +56     
==========================================
+ Hits         3140     3146       +6     
- Misses        728      778      +50     

Files	Coverage Δ
src/scirpy/pp/_index_chains.py	53.70% <83.05%> (-46.30%)	⬇️

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