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This class, in combination with `TCForall`, turns out the useful in
LambdaRust to express that lists of expressions are values.

This commit is based on code by Amin Timany.

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

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

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

This closes issue #64.

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.

Replace/remove some occurences of `persistently` into `persistent` where the property instead of the modality is used.

Rename `UCMRA` → `Ucmra`
Rename `CMRA` → `Cmra`
Rename `OFE` → `Ofe` (`Ofe` was already used partially, but many occurences were missing)
Rename `STS` → `Sts`
Rename `DRA` → `Dra`

I have reimplemented the tactic for introduction of ∀s/pures using type
classes, which directly made it much more modular.