## os.rkt and friends

 - move from quasiquoted to prefab structs
 - define a prefab struct for every evt? that we want to use
 - define lowest-level drivers for each prefab struct

It feels like those lowest-level drivers are listening for *demand*
i.e. they're listening for presence and are then acting to supply such
demand. Think about the relationships between presence (both positive
and negative), and interfacing to ad-hoc sources and sinks.

### Old, possibly-still-relevant TODOs from os.rkt

 - is timeout really primitive? If so, isn't presence primitive?
 - what about metatimeout?
 - enforce user-mode restrictions