Remove num-derive dependency

[FreezyLemon]

I checked that the implementation is equivalent with cargo expand.

This change removes syn from the build with default features and reduces compile times by roughly 50% on my machine. I think that's good enough to warrant ~15 lines of boilerplate.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.30%. Comparing base (f0c70fd) to head (4738a3a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #32      +/-   ##
==========================================
+ Coverage   67.22%   68.30%   +1.07%     
==========================================
  Files           4        4              
  Lines         949      978      +29     
==========================================
+ Hits          638      668      +30     
+ Misses        311      310       -1     

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[FreezyLemon]

I guess I should add tests while I'm at it

[lu-zero]

I guess it needs a rebase but looks fine to me.