add a strong form of atomicity, for weak forms of weakest-pre

Closes #107

See merge request FP/iris-coq!77

This way, it can be used with `iApply`.

Notation for disjointness: replace ⊥ with ##, so that ⊥ can be used for bottom.

See merge request FP/iris-coq!73

This is to be used on top of stdpp's 4b5d254e.

Instead, as Ralf suggested, just comment the test out.

This closes issue #64.

The plainness modality

See merge request FP/iris-coq!71

Now that we have the plain modality, we can get rid of the basic updates
in the soundness statement.