Add support for manually serializing X·M·L

This commit adds a transform for a new `<书社:serialize-xml>` element,
which is useful in conjunction with `<书社:raw-text>` to produce a more
finely‐controlled X·M·L output, or in other X·M·L‐y situations where an
escaped X·M·L value is required. The algorithm used for serialization
attempts to closely match the DOM Parsing and Serialization spec,
including such behaviours as mandating an undeclared `xml:` prefix for
the X·M·L name·space and dropping the prefix from elements whose
name·space matches the default, but it probably isn’t exactly the same
(due in part to the fact that the underlying data structure is an X·M·L
infoset, not a potentially dynamically‐modified Dom). No special
allowances are made for elements in the H·T·M·L name·space; this is not
(yet) a suitable polyglot serializer (or intended to be one).