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.

For half of #231.

This was a noop and will soon be an error (until `Inductive` properly
supports locality attributes). See https://github.com/coq/coq/pull/9410