Visualisation improvement: role variables get priority in assigning concrete values.

Previously, if a local (non-role) variable would have (implicit) agent type,
it would be assigned a concrete term before the roles. This would lead to
non-optimal choices, since we care more about the role instantiations than about other variables.

However, when making traces concrete, we use the run's 'locals' list. Because of
the way this is constructed (in reverse), non-role variables precede the role variables.
We therefore choose to traverse the list in reverse.