feat+refactor: Bump MSRV to 1.57 and maximize const
          #296
        
          
      
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Similar to #294, except it puts the changes behind a feature flag
constand uses a macroconst_fnto avoid code duplication. This allows the MSRV to stay low (still had to go up to1.57because I was getting weirdserdeerrors, but if someone can figure those out, it might be possible to avoid the MSRV bump)