Nicer soundness statements for the base_logic.

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.