Allow for self-referential subviews in the constraint props array

[tptee]

It's redundant that we have to specify the current subview name in the constraint builders:

<AutoDOM.div
  name="tutorialSubview"
  intrinsicWidth={50}
  intrinsicHeight={50}
  constraints={[
    constrain().subview("tutorialSubview").centerX
      .to.equal.superview.centerX,
    constrain().subview("tutorialSubview").centerY
      .to.equal.superview.centerY
  ]}
 >
  This is a subview
</AutoDOM.div>

This is preferable (still allowing references to other subviews):

<AutoDOM.div
  name="tutorialSubview"
  intrinsicWidth={50}
  intrinsicHeight={50}
  constraints={[
    constrain().thisSubview.centerX
      .to.equal.superview.centerX,
    constrain().thisSubview.centerY
      .to.equal.superview.centerY
  ]}
 >
  This is a subview
</AutoDOM.div>