Retrieving all Context components from outer nested iteration scopes

The *Instance Index* node has a *Loop Level* parameter that lets you access the outer scopes of multiple levels of nested iteration. This is akin to `i`, `j`, and `k` index variables commonly used in nested `for` loops in programming.

Today, I ran into the need to access an outer nested iteration scope with the *Instance Position* node as well. More generally, it needs to be possible to access any component of the `Context` (`OwnedContextImpl`) by iterating from inwards to outwards and accessing the outer levels. This likely needs to involve @TrueDoctor deciding on some architectural decisions for Graphene to make this possible.