Replace all instances of 'self' with 'static' (#24)

Fixes #22

Author: vikas020807

src/IsBackedEnum.php

@@ -25,27 +25,27 @@ protected static function ensureImplementsInterface(): void
     public static function options(): array
     {
         static::ensureImplementsInterface();
-        return array_map(fn($enum) => $enum->toArray(), self::cases());
+        return array_map(fn($enum) => $enum->toArray(), static::cases());
     }
 
     public static function names(): array
     {
         static::ensureImplementsInterface();
-        return array_map(fn($enum) => $enum->name, self::cases());
+        return array_map(fn($enum) => $enum->name, static::cases());
     }
 
     public static function values(): array
     {
         static::ensureImplementsInterface();
-        return array_map(fn($enum) => $enum->value, self::cases());
+        return array_map(fn($enum) => $enum->value, static::cases());
     }
 
     public static function map(): array
     {
         static::ensureImplementsInterface();
         $array = [];
 
-        foreach (self::cases() as $enum) {
+        foreach (static::cases() as $enum) {
             $array[$enum->value] = $enum->label();
         }
 
@@ -55,7 +55,7 @@ public static function map(): array
     public static function labels(): array
     {
         static::ensureImplementsInterface();
-        return array_map(fn($enum) => self::labelFor($enum), self::cases());
+        return array_map(fn($enum) => static::labelFor($enum), static::cases());
     }
 
     public static function labelFor(self $value): string