Add `Scope`-> `GroupName` mapping

[leonqadirie]

Relating to #177 (comment):

This is where having another index might be helpful, i.e. Scope -> Group name in a separate mapping. But that's an optimization that can be added in the future.

To make sure I understand you correctly before tackling this:
Are we talking about sth. along the lines of substituting ScopeGroupKey with sth. like this?

pub struct Scope {
    name: ScopeName,
    groups: HashSet<GroupName>,
}

[slawlor]

Sorry again on the delay getting back to you. I meant another global index linking the scopes to groups. So you could efficiently lookup the groups from a given scope. This would remove some scan operations but it is a minimal operation.

[leonqadirie]

No problem, again, take all the time required. Hope all is fine and you get some sleep ;)
Will open a PR with what I understood this to be.

[slawlor]

#190 resolved this.