Embed nesting information in Keys

Currently, a `Key` is mainly a typed label without information about the position of the referenced object within a hierarchy of nested objects.
It would be useful to have `Key`s embed this information, so that, for instance, when writing to `register("B")` which was retrieved from `map_aw("A")` it doesn't write to a top-level register `B` but to its original, nested one.