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.
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