Make deletion vector manipulation less error-prone

[scovich]

Deletion vectors in a scan result can be truncated, with fewer entries than the corresponding engine data.

Remove a footgun by making the raw DV in a ScanResult private, with new accessors to make it less error-prone to use.

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.

Project coverage is 76.56%. Comparing base (0fdb40a) to head (52ac699).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
kernel/src/scan/mod.rs	88.88%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #374      +/-   ##
==========================================
+ Coverage   76.54%   76.56%   +0.02%     
==========================================
  Files          47       47              
  Lines        9384     9393       +9     
  Branches     9384     9393       +9     
==========================================
+ Hits         7183     7192       +9     
  Misses       1799     1799              
  Partials      402      402              

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[zachschuermann]

lgtm thanks ryan!!

[zachschuermann]

very nice! I like this better than extend(iter::repeat().take())

[zachschuermann]

flagged as breaking since we are taking the mask private

[nicklan]

nice! thanks, this is better than adjusting it everywhere.