This PR migrates the ⌜p⌝ notation for embedding pure p : Prop into SPred
σs to expand into a simple, first-order expression SPred.pure p that can
be supported by e-matching in grind.
Doing so deprives ⌜p⌝ notation of its idiom-bracket-like support for
#selector and ‹Nat›ₛ syntax which is thus removed.