categorizing traits objects

[Urs Hoelzle]

As a followup to John's mail, I'd like to point out that method
categories are expressed with "slot annotations" in the new system.
That is, we no longer use parents to factor traits objects.  Instead,
the traits object contains all methods at once.  The slots are grouped
into categories by adding annotations (meta-information). This
information is accessible through mirrors.

-Urs