For reasons that AFAIK are no longer relevant, there were two
implementations of XPath generation for use in serializing range
selectors. Since we're removing jQuery from the Hypothesis client, this
removes the jQuery implementation and simplifies the remaining DOM-only
one.

 - Remove jQuery XPath generation

 - Change the non-jQuery implementation to take DOM Nodes as input
   rather than jQuery collections

 - Simplify and add type documentation for the remaining implementation

 - Correct the documentation for `xpathFromNode`. The previous comments
   said that it _evaluated_ XPaths but actually it _generates_ them.