Implements some micro-optimizations

PkmnLib.Static/Species/Species.cs

@@ -64,7 +64,7 @@ public interface ISpecies : INamedValue
     /// <summary>
     /// Check whether the Pokémon has a specific flag set.
     /// </summary>
-    bool HasFlag(string key);
+    bool HasFlag(StringKey key);
 
     /// <summary>
     /// The data regarding into which Pokémon this species can evolve, and how.
@@ -147,5 +147,5 @@ public class SpeciesImpl : ISpecies
     }
 
     /// <inheritdoc />
-    public bool HasFlag(string key) => Flags.Contains(key);
+    public bool HasFlag(StringKey key) => Flags.Contains(key);
 }