Description:
------------
Currently, const defined in classes are always public. I suggest allow private/protected it. It'll allow I define my own private/protected constants inside my class, without need create a static private/protected property to store this kind of data.

Currently:

    class Test 
    {
        private static $VERSION_MATCH = '/^\d+\.\d+$/';
        public static function match($version) 
        {
            return preg_match(self::$VERSION_MATCH, $version);
        }
    }

After implementation:

    class Test 
    {
        private const VERSION_MATCH = '/^\d+\.\d+$/';
        public static function match($version) 
        {
            return preg_match(self::VERSION_MATCH, $version);
        }
    }

Imagine that I don't want that this const be public.

    class_statement:
        variable_modifiers property_list ';'
 ~    | class_member_modifier T_CONST class_const_list ';'
      | T_USE name_list trait_adaptations
      | method_modifiers function returns_ref identifier '(' parameter_list ')'
            return_type backup_doc_comment method_body;

 +  class_member_modifier:
 +      /* empty */
 +    | T_PUBLIC
 +    | T_PROTECTED
 +    | T_PRIVATE
 +    | T_FINAL;