Yield

[thekid]

This pull request adds the yield expression to XP language and PHP syntax.

class Test {
  private static var values() {
    yield 1;
    yield 2;
  }

  private static var mappings() {
    yield 'number' : 1;
    yield 'color' : 'green';
  }

  // ...
}

To support it by default, the PHP 5.5 emitter becomes the default. To downgrade to emit PHP 5.4 code, use -E php5.4.

Note: This will currently raise an error since PHP 5.4 doesn't support generators, and we cannot simulate them side-effect freely without language support!