[layout] Move Kani proofs from crate root

[joshlf]

All moved proofs test code from the layout module, and so belong there
rather than in the crate root.

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This PR is on branch cell-traits.

• Teach transmute_{ref,mut}! to handle slice DSTs #2428
• [layout] Move Kani proofs from crate root #2430

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.53%. Comparing base (04cafb2) to head (3baab3f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2430   +/-   ##
=======================================
  Coverage   87.53%   87.53%           
=======================================
  Files          18       18           
  Lines        6408     6408           
=======================================
  Hits         5609     5609           
  Misses        799      799           

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