Rose::HTMLx::Form::Field::Serial is a subclass of
Rose::HTML::Form::Field::Hidden. It exists simply to isolate a
particular kind of form field that should not be updated via form
but may need to be passed as a param or viewed in a (x)html serialized
format. The namespace is reserved in the event that future
functionality may be added, but mostly to uniquely identify this
field type for use with Rose::DBx::Garden.