The core of anchoring in the guest code is now asynchronous.

- Anchoring annotations happens asynchronously in the setup function.

- Creating an annotation is asynchronous, and depends upon the resolution
  of the targets, which may depend on the resolution of the document metadata.
  The first events fire only after this happens.

- The Anchor API can now optionally return Promises when converting ranges
  and selectors, though all the present implementations are synchronous.