Rework AWS access as actual "object hierarchy"

[danfuzz]

As an "and then" of issue #37, maybe it's reasonable to want to treat all of AWS as an "object hierarchy," with each object exposing properties and methods. E.g. maybe the root is a container of
Region objects, leading to (among other things) Zones, leading to (among other things) Instances, etc.

Probably more trouble than it's worth to flesh it out completely, but it might make sense to implement this piecemeal as needed by clients.