feat: `grind` proof parameters whose type is not a `forall`

This PR ensures that users can provide `grind` proof parameters whose
types are not `forall`-quantified. Examples:

```lean
opaque f : Nat → Nat
axiom le_f (a : Nat) : a ≤ f a

example (a : Nat) : a ≤ f a := by
  grind [le_f a]

example (a b : α) (h : ∀ x y : α, x = y) : a = b := by
  grind [h a b]
```