This PR implements `instantiateRevBetaS`, which is similar to
`instantiateRevS` but beta-reduces nested applications whose function
becomes a lambda after substitution.
For example, if `e` contains a subterm `#0 a` and we apply the
substitution `#0 := fun x => x + 1`, then `instantiateRevBetaS` produces
`a + 1` instead of `(fun x => x + 1) a`.
This is useful when applying theorems. For example, when applying
`Exists.intro`:
```lean
Exists.intro.{u} {α : Sort u} {p : α → Prop} (w : α) (h : p w) : Exists p
```
to a goal of the form `∃ x : Nat, p x ∧ q x`, we create metavariables
`?w` and `?h`. With `instantiateRevBetaS`, the type of `?h` becomes `p
?w ∧ q ?w` instead of `(fun x => p x ∧ q x) ?w`.
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