Refactor the functions that run over every event. Now there are appropriate "for each event", "for each fd", and "for each signal" helpers that they can use; this makes the code a bit simpler, and far less duplicated. This lets me turn back on the functions I disabled when removing eventlist. Additionally, check more lists for circularity in event_base_assert_ok(). Add typedefs for the callback types. Name fewer things "ctx". Adds an implementation of Floyd's tortoise-and-hare algorithm to check for circularity in TAILQs and LISTs, to avoid the abuse of flags that event_base_assert_ok() was doing before. Suggested by Dave Hart.