Relicense to MIT, support the attribute spread operator

This release:

• is available under the MIT license
• adds support for the XHP spread operator (available in HHVM 3.24+)

The XHP Spread Operator

This is intended to be a safer replacement for the implicit attribute copying available via XHPHelpers; we expect to remove XHPHelpers from XHP 3.0.

class :inner extends :x:element {
  attribute string color, int age;
}

class :outer-old extends :x:element {
  use XHPHelpers;

  attribute string color, int age;

  public function render(): :xhp {
    // `age` attribute is implicitly copied
    return <inner color="red" />;
  }
}

class :outer-new extends :x:element {
  attribute string color, int age;

  public function render(): :xhp {
    // Explicitly copy all attributes from `$this` - however, the override of color takes precedence
    return <inner {...$this} color="red" />;
  }
}

The typechecker is able to validate types - for example:

class :outer-invalid extends :x:element {
  attribute string color, float age;

  public function render(): :xhp {
    // Type error:
    // Invalid xhp value for attribute :age in :inner
    //   This is an int
    //   It is incompatible with a float
    return <inner {...$this} />;
  }
}