This commit introduces the following soundness statements:

- Soundness of pure propositions `⌜ φ ⌝%I → φ`.
- Soundness of later `(▷ P)%I → P`.

The old soundness statement, `(▷^n ⌜ φ ⌝)%I → φ` is now a derived form.

This is in preparation for coq/coq#9274.

This lemma is similar to `later_ownM`.

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

Use explicit names in some scripts, re-organize fupd plainly derived laws, adjust wsat import/export.

Modify adequacy proof to not break the 'fancy update' abstraction. Modify fupd plainly interface and add new derived results.

With a pretty proof by Robbert

The BI interface is then instantiated using these laws.  In particular, this
shows that the rules we claim to be admissible actually are.

Fixes #193

Thanks to @dfrumin.