Run a callback when the interruption epoch is reached

[SquidDev]

Adds Store::epoch_deadline_callback. This accepts a callback which, when invoked, can mutate the store's contents. The callback can either return an error (in which case we trap) or return a delta which we'll use to set the new epoch deadline.

As discussed with @alexcrichton on Zulip. I'm not very happy with the tests here - happy to improve those if people have ideas.

It might be desirable to reimplement the trap and yield behaviours in terms of this callback. Happy to do that if preferred.

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[alexcrichton]

Looks good to me, thanks! Could you add a test which also returns a trap from the closure? Finally could you also add a test which specifically uses this function and asserts that the callback was indeed called? (e.g. frobs the state on T or something like that)

[SquidDev]

Thank you so much!