refact!: require unsafe for Relax

[pedromfedricci]

Breaking changes:

• mcslock::relax::Relax is now properly marked as a unsafe trait. There is a safety invariant that implementers must satisfy that was previously overlooked and undocumented. Relax functions must not exit the thread, doing so is undefined behavior.
• mcslock::relax::Relax no longer requires that types must also implement Default.
• mcslock::relax::Relax now requires implementing the new function, which is now used to create the initial value. Previously the value that was created by a type's Default::default implementation.
• All relax types under mcslock::relax::* no longer implement Default.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.51%. Comparing base (aceb758) to head (3ef9f61).

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #14      +/-   ##
==========================================
+ Coverage   99.48%   99.51%   +0.02%     
==========================================
  Files           5        5              
  Lines         195      205      +10     
==========================================
+ Hits          194      204      +10     
  Misses          1        1              

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