[byte_slice] Add Kani proofs

[joshlf]

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This PR is on branch ptr-kani.

• [kani] Prove some PtrInner methods #2444
• [byte_slice] Add Kani proofs #2451

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.43%. Comparing base (855c237) to head (19ae290).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2451   +/-   ##
=======================================
  Coverage   90.43%   90.43%           
=======================================
  Files          18       18           
  Lines        7331     7331           
=======================================
  Hits         6630     6630           
  Misses        701      701           

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