refactor FlexMF with less string compares

[mdekstrand]

This refactors FlexMF's tight loops to reduce string comparisons.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.03%. Comparing base (08d2b94) to head (eebe122).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #752      +/-   ##
==========================================
- Coverage   90.17%   90.03%   -0.14%     
==========================================
  Files         138      138              
  Lines        9799     9813      +14     
==========================================
- Hits         8836     8835       -1     
- Misses        963      978      +15     

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