Rename "proc"/"pump" to "driver" and update format

[kjekac]

Procs should be called drivers. Additionally we should change their format from this

type Driver = Array (MsgID, Effect) -> MsgID -> Any -> Driver

to this

type Driver = MsgID -> Any -> (Driver, Array (MsgId, Effect))

Since this is more consistent with the format for cogs, where outputs aren't kept in the closure. The new docs (WIP; not yet published) already reflect this format change.