Mark toArray as pure and force psalm to accept it

Lie to Psalm and tell it that toArray is immmutable. This seems tolerable (despite how bad a practice overriding the typechecker is) because the mutation is entirely memoization. If PHP were a lazy language, no caching would be necessary in the first place.

src/Enum.php

@@ -86,6 +86,7 @@ public function getKey()
 
     /**
      * @psalm-pure
+     * @psalm-suppress InvalidCast
      * @return string
      */
     public function __toString()