Move masking/filling to dedicated struct

[femshima]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 97.77778% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 68.61%. Comparing base (2a4f396) to head (5be473e).

Files	Patch %	Lines
src/sequence.rs	96.77%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #27      +/-   ##
==========================================
+ Coverage   68.59%   68.61%   +0.01%     
==========================================
  Files          30       31       +1     
  Lines        1554     1558       +4     
==========================================
+ Hits         1066     1069       +3     
- Misses        488      489       +1     

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