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.

These results turned out to be neither that useful nor canonical, and can easily
be derived from local updates.

This reverts commit 465dd9f4.