In same spirit as the other 'primitive' types like `option`, `prod`, ...

This is an old flag set by the ssr plugin, and recently unset in coq-stdpp,
see https://gitlab.mpi-sws.org/robbertkrebbers/coq-stdpp/issues/5.

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

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.

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`

For obsolete reasons, that no longer seem to apply, we used ∅ as the
unit.

That way, we can use Contractive as part of structure definitions in
which we do not have a bundled OFE yet.