Show that atomic_step is an AccElim so we can open invariants; get entirely rid of the Em ⊆ Eo side-condition

New atomic updates: defined as a fixed point with existential quantifier; intro lemma using class of Laterable assertions

If the BI is not affine, this should not happen, as it may lead to
information loss.

This commit fixes issue #190.

Thanks to @dfrumin.

These instances are the same as those for e.g. the later modality.

provide big_op lemmas outside of bi module

See merge request FP/iris-coq!149

mktemp seems to also be available on macOS -- though without `--suffix`.

There's a very low risk of these conflicting with Coq's standard library

This version allows one to either close or cancel the invariant after opening it.