Add API to make juggling Gee collections easier

This adds a simple Iterable class that lets us take advantage of Gee's
Traversable interface much more easily.  Traversable is great, but
every operation returns an Iterator, which makes it awkward to use
outside of Traversable.  The new Iterable wraps the Traversable
Iterators and methods so you can directly use the result.  It also gives
us a convenient point to add convenience methods in the future.

I've gone through a few arbitrary places in the code to see how the
class might be used, and changed some obvious places to (hopefully) the
equivalent code using the new Iterable class.  More work could be done
here, but the real benefit is simply having the Iterable class around to
be able to use in new code.