Attempt at changing ItemId struct into a trait, and passing ItemIdT type parameter around.

[azriel91]

Closes #158.

This doesn't actually implement it -- the number of types that need the <ItemIdT> type parameter is large, and the API ergonomic hit, as well as framework maintenance is so costly that this approach is not desirable.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (3683183) 93.85% compared to head (8edea31) 93.85%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #181   +/-   ##
=======================================
  Coverage   93.85%   93.85%           
=======================================
  Files         407      407           
  Lines       41320    41320           
=======================================
  Hits        38780    38780           
  Misses       2540     2540           

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