Suppress deprecation warnings in generated code

[google-labs-jules]

Fixes an issue where deriving zerocopy traits on deprecated structs would emit warnings about "use of deprecated item".

This PR adds #[allow(deprecated)] to:

• derive_known_layout's Field implementations.
• derive_known_layout's internal MaybeUninit struct.
• derive_known_layout's KnownLayout impl for the internal struct.
• derive_hash implementation (was missing).
• derive_eq implementation (was missing).

It also adds regression tests in zerocopy-derive/tests/deprecated.rs which define deprecated types and derive various traits, ensuring no warnings are emitted (the tests have #![deny(deprecated)]).

Closes #553

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PR created automatically by Jules for task 3723672898041436653 started by @joshlf

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.14%. Comparing base (a9e527a) to head (9c1f7b7).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2797   +/-   ##
=======================================
  Coverage   88.14%   88.14%           
=======================================
  Files          20       20           
  Lines        5450     5450           
=======================================
  Hits         4804     4804           
  Misses        646      646           

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