* TODOs
  
** Breaking

*** TODO Why does ~add_pid~ take a ~filter~
*** TODO Don't expose all of the constants from ~kqueue-sys~
*** TODO Express which filters are appropriate for which functions in the type system

** Non-breaking

*** TODO Why does remove_* call ~kevent~ and not add_*?
*** TODO Try and abstract away ~NOTE_TRACK~ and ~NOTE_CHILD~ if possible and non-breaking
*** TODO ~get_event~ should probably check ~started~ status
*** TODO Make sure that filters and flags are sensible
*** TODO Why does ~as_size~ exist?
*** TODO implement ~timer~ and ~signal~
*** TODO ummm add ~stop~?
*** TODO iterator with a timeout?