Reflex doesn't have a Pure Time module, Reflex.Time is impure #534

The Reflex.Time module uses the impure PerformEvent class. As a consequence, using Reflex.Time functions for e.g. animations would lead to glitches, e.g. if multiple identical animations start simultaneously.

We should:

• Make the impure nature of the module clear in the documentation.
• Consider renaming the module appropriately.
• Implement a pure version of Time.

An efficient implementation of a pure Time would need a priority queue containing "scheduled time" values. In every frame, if the queue contains a value, check the current system time T; if scheduled time <= T, fire the triggers simultaneously.

Users might also want access to a time behavior. Implementing this behavior correctly and efficiently, together with the "scheduled time" queue is somewhat tricky, but must be possible.

A minimal class could look like this:

class (Reflex t) => HasTime t m | m -> t where
  type Time t :: Type
  type PositiveTimeDelta t :: Type
  time :: Behavior t Time
  after :: PositiveTimeDelta t -> m (Event ())