v2 RC1 Changes (#7)

- Changes `items` and `hasItems` on `DocumentList` to be properties
- Updates dependent option/result library, which contains similar changes

Reviewed-on: #7

tests/unit/Mapper/ArrayMapperTest.php

@@ -22,7 +22,7 @@ class ArrayMapperTest extends TestCase
     public function testMapSucceeds(): void
     {
         $result = ['one' => 2, 'three' => 4, 'eight' => 'five'];
-        $mapped = (new ArrayMapper())->map($result);
+        $mapped = new ArrayMapper()->map($result);
         $this->assertSame($result, $mapped, 'The array mapper should return the parameter given to it');
     }
 }