Nested objects extract

This is a draft of a nested object extract feature by assigning the object => hydrator key and value pair to renderer/hydrators map.
I would move this into a recursive method, actually. Right now, you're assuming that nesting only happens up to 2 levels deep, when depth can be arbitrary.

This is also a potential rationale for using hydrator strategies instead, as those allow us to do nested objects in the first pass, instead of requiring recursion here.