This is handy so that we don't have to keep multiple objects around to
handle this request. Otherwise we'd need to keep references to the
document and that is a bit annoying.
This of course has a limit of about an hour (in tv_nsec), but that is
far longer than we can realistically record anyway.
We want to have an object hierarchy, but we don't want to expose our class
or other internal details to the public API/ABI. This uses the old form of
class definition so we can maintain that.
An alternative would be to do what GDK does for internal types, should we
find that easier to maintain going forward.
Also handle swapping the data when non-native endian so that we can leave
our GMappedFile unperturbed (and therefore not have to keep those bytes as
dirty pages in process).
