Improve bitmap slice unchecked

[Dousir9]

if a Bitmap is all true or all false, we don't do a bitcount in slice_unchecked.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (6271f48) 83.06% compared to head (7af8160) 83.05%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1574      +/-   ##
==========================================
- Coverage   83.06%   83.05%   -0.02%     
==========================================
  Files         391      391              
  Lines       42898    42901       +3     
==========================================
- Hits        35635    35631       -4     
- Misses       7263     7270       +7     

Files	Coverage Δ
src/bitmap/immutable.rs	85.58% <100.00%> (-0.26%)	⬇️

... and 3 files with indirect coverage changes

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