Decreased boilerplate for XHPAsync

Using the XHPAsync now implicitly marks your class as implementing XHPAwaitable.

Before:

class :foo extends :x:element  implements XHPAwaitable {
  use XHPAsync;
}

After:

class :foo extends :x:element {
  use XHPAsync;
}

You can still explicitly mark your classes as implementing XHPAwaitable if you prefer, or for backwards compatibility.