Allow a command executor to be able to preview what permissions they'd have working from the perspective of a different subject.

I'm thinking of this as a testing feature, though input on how this would be useful in gameplay would be appreciated as well and could lead to expansions of this feature.

Things to consider:

• su-ing to a role with higher permissions, is it worthwhile trying to restrict that?
• how to handle writes to the original subject in "su mode" -- proxy as well, block, or write to the original subject?
• perhaps we check the inheritance tree of the targeted subject, and block su-ing if the original subject has su permission explicitly denied for any of the subject's parents
• some indication/audit logging of these changes?