Rewrite the `Guest#anchor` method to simplify the control flow and
generally make it easier to understand and change. There are no
functional changes for annotations with zero or one entry in the
`target` field (all existing Hypothesis annotations).

There is a functional change to handling of annotations with multiple
targets. Previously `anchor` would try to be smart about only
re-anchoring targets which were not already anchored. In the new
implementation all targets for an annotation are re-anchored. This will have
no effect in practice because the Hypothesis client only supports
creating annotations with a single target and the h backend can only
store a single target per annotation. However the Hypothesis API does
allow for multiple targets per annotation, in line with the W3C Web
Annotations specs in which a single annotation can refer to multiple
parts of a document.

 - Convert `anchor` method to async and replace Promise chains with async/await

 - Replace the logic that removes existing anchors and highlights for an
   annotation with a call to the `detach` method. This required adding
   an internal parameter to `detach` to control whether `anchorsChanged`
   is emitted, so that `anchor` only emits `anchorsChanged` once.

 - Add an explicit error to `Promise.reject` calls in tests so that the
   tests are easier to debug if they fail