This is in preparation for coq/coq#9274.

This lemma allows one to get the witness out of a later, without having
to use `later_car`, i.e. in a way that works in the on-paper version of
the logic.

Adding a hint without a database now triggers a deprecation warning in
Coq master (https://github.com/coq/coq/pull/8987).

This is backwards-compatible; it desugars to a normal application on previous versions

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

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`

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

Now, we never need to unfold LimitPreserving in LambdaRust, and hence
the entails_lim tactic is no longer needed.