ActionController filters provide a way to register callbacks that run
before, after or around an action (i.e. HTTP request handler). They run
in the same class context as the action, so can get/set instance
variables and generally interact with the action in arbitrary ways.
In order to track flow between filters and actions, we have to model the
callback chain. This commit does that. A later change will add dataflow
steps to actually track flow through the chain.
