core: Don’t modify a HashSet while iterating over it

Even though we were modifying it from the Iterator (which should’ve been OK),
we got hit by https://bugzilla.gnome.org/show_bug.cgi?id=671327 which meant
that some Personas weren't getting disconnected from Individuals. This is a
likely cause of bug #670523.

This also includes a related (but irrelevant to the bug itself) tidy-up to
Persona._individual.

Closes: https://bugzilla.gnome.org/show_bug.cgi?id=670523