Factor out struct/enum variant is_safe validation into a shared helper

[jswrenn]

---

• 　  Implement common traits for ReadOnly #3682
• 　  Introduce unstable #[zerocopy(invariant(...))] library invariant checking #3681
• 👉 Factor out struct/enum variant is_safe validation into a shared helper #3680
• 　  Rename Ptr::try_into_valid to Ptr::try_into_safe #3679

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.90%. Comparing base (e97c630) to head (be894c6).

Additional details and impacted files

@@                            Coverage Diff                             @@
##           G43414d79608bdfefacdf84890ce8f2224bdb5ceb    #3680   +/-   ##
==========================================================================
  Coverage                                      91.90%   91.90%           
==========================================================================
  Files                                             20       20           
  Lines                                           6175     6175           
==========================================================================
  Hits                                            5675     5675           
  Misses                                           500      500           

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[jswrenn]

@joshlf This is just a preamble to #3681 to keep that diff smaller.