Object-based traversal

Here's a branch containing the object-based traversal subclass I'm using in my falcon branch of Mailman 3.  While probably not a full fidelity port of the restish API, it's close enough that my entire test suite passes.

For the most part, I'm PR'ing this to start a conversation about how and whether object-based traversal can be made a first class citizen of falcon.

Note that there are a number of other little helpers and utilities I'm using on top of falcon, but they're independent of the basic object-based traversal support, so I'll leave those off this branch and discuss them separately.