* Run a callback when the interruption epoch is reached
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.
* Add a basic test for epoch interruption callback
* Some small nits
- Remove use of &mut in the pattern match
- Return both yields and state from run_and_count_yields_or_trap in
test code and assert on them separately.
- Add a test for trapping on a state failure.
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