Retry library.

3 main states

* success (all good)
* error (bad, potentially retry)
* failure (terminal failure)

- Conditions control if we should retry and how to setup the state
at first call, and how to update the state upon failures (inc retry
counter for instance). Conditions are in effect, composable and
should remain pure (state in/out).

You can for instance do:

;; will retry up to 3 times with a 1000ms delay between tries
(cm/do! #(do-something) [(delayed-retries (constant-backoff-delays 1000)) (max-retries 3)])

- Effects can apply side effects at various stages (ex sleep, update
state machine), they are purely for io, as opposed to Conditions