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.
This PR implements the option `mvcgen +jp` to employ a slightly lossy VC
encoding for join points that prevents exponential VC blowup incurred by
naïve splitting on control flow.
```lean
def ifs_pure (n : Nat) : Id Nat := do
let mut x := 0
if n > 0 then x := x + 1 else x := x + 2
if n > 1 then x := x + 3 else x := x + 4
if n > 2 then x := x + 1 else x := x + 2
if n > 3 then x := x + 1 else x := x + 2
if n > 4 then x := x + 1 else x := x + 2
if n > 5 then x := x + 1 else x := x + 2
return x
theorem ifs_pure_triple : ⦃⌜True⌝⦄ ifs_pure n ⦃⇓ r => ⌜r > 0⌝⦄ := by
unfold ifs_pure
mvcgen +jp
/-
...
h✝⁵ : if n > 0 then x✝⁵ = 0 + 1 else x✝⁵ = 0 + 2
h✝⁴ : if n > 1 then x✝⁴ = x✝⁵ + 3 else x✝⁴ = x✝⁵ + 4
h✝³ : if n > 2 then x✝³ = x✝⁴ + 1 else x✝³ = x✝⁴ + 2
h✝² : if n > 3 then x✝² = x✝³ + 1 else x✝² = x✝³ + 2
h✝¹ : if n > 4 then x✝¹ = x✝² + 1 else x✝¹ = x✝² + 2
h✝ : if n > 5 then x✝ = x✝¹ + 1 else x✝ = x✝¹ + 2
⊢ x✝ > 0
-/
grind
```
This PR adds support in the `mintro` tactic for introducing `let`/`have`
binders in stateful targets, akin to `intro`. This is useful when
specifications introduce such let bindings.
Closes#9365.
