Make Rbac iterable

[encody]

This could mean:

• Changing the current Rbac to support role holder enumeration. This would be a state-breaking change.
• Adding an EnumerableRbac alternative that supports enumeration.

[encody]

An EnumerableRbac could be implemented in two ways:

• Implementing the whole iterable collection manually
• Just serializing all of the AccountIds into a single slot. Though this is usually frowned upon, in this case it might not actually be the worst idea: The collection has to be iterable, and a smart contract should rarely, if ever, iterate over a collection that is very large, due to gas concerns. This would require some testing.