Add bindings to XMLFragment, XMLElement, XMLText, and XMLEvent. Allows
reading and writing of XML fragments from the document, which rich text
editors use heavily.

Closes jupyter-server#151

Rich text editors often store their markup in attributes and embeds.

Currently trying to use apply_update in a transaction causes a deadlock,
as apply_update tries to make its own new transaction but is locked out.
This reuses the existing transaction, which also allows applying
multiple updates in a single transaction.

Allows comparing whether objects refer to the same element in a doc.
Important since every access generates a new, unique Python wrapper
instance.

__eq__ is based directly from the underlying rust objects' `Eq`
implementation. __hash__ is based off of the `Hash` implementation of
the `BranchID` of the rust objects.

Like Text, XmlText can contain embedded formatting, so make that
available.