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.

Coq also uses level 200 for these constructs.
Besides, heap_lang's match and if were also already at this level.

Now, associativity needs only to be established in case the elements are
valid and their compositions are defined. This is very much like the notion
of separation algebras I had in my PhD thesis (Def 4.2.1). The Dra to Ra
construction still easily works out.