Propose addition of a SubjectRef element

[Scott Cantor <cantor.2@osu.edu>]

Before the issue hangs around any longer, I'd like to propose a solution to
the issue of optimizing the carriage of multiple statements that share a
subject in a single assertion by proposing that we add the ability for a
SubjectStatement to include a Subject element by reference in addition to by
value.

This is much more than just a space saver in the case where a non-trivial
SubjectConfirmation is used, since duplicating the confirmation and then
possibly evaluating it twice is not good.

My proposed solution is that we add an ID attribute to SubjectType and add a
new element/type called SubjectRef that just includes an ID attribute (of
type IDREF). We then allow SubjectStatement to carry either Subject or
SubjectRef.

E.g.

<AuthenticationStatement>
	<Subject ID="sub1"><NameIdentifier>...</NameIdentifier></Subject>
</AuthenticationStatement>
<AttributeStatement>
	<SubjectRef ID="sub1"/>
	<Attribute/>...
</AttributeStatement>

I'm open to whatever stylistic choices people want (make SubjectRef contain
the reference as element content, etc.).

I believe this will be easier spec-wise than trying to create the notion of
a "default" subject in the current data model and this should solve the same
set of problems.

-- Scott