There are situations where Firefox creates a selection with multiple
ranges, even though the spec forbids this. In various places in our code
we called `selection.getRangeAt(0)` assuming there would only be one
range. In the case where FF had created multiple ranges, this led to
only part of the user's selection being respected.

 - Modifying the `selectedRange` utility to combine multi-range
   selections into a single Range, so that the results in Firefox are
   the same as in other browsers.

 - Replacing various direct calls to `selection.getRangeAt(0)` with
   calls to `selectedRange` so that they take get this normalization
   applied.

   In the tests for `Guest`, this led to refactoring to make some tests
   less tied to implementation details.

Fixes https://github.com/hypothesis/client/issues/5485