I'm starting to convert the show formats to the new mime_node_t
self-recursive style.  Before I go through all of them, I wanted to
get some feedback.

Besides general feedback, I'm interested in knowing if this is a good
way to structure the patches for review.  Patches 1 through 3 just get
requirements out of the way, but the actual conversion is done in two
steps: patch 4 just moves the existing code into the new code
structure without changing any of the logic and patch 5 simplifies the
restructured code to actually benefit from the new structure.  The
hope is that patch 4 is easy to review even though it's big and that
patch 5 makes the real (and interesting) changes obvious.