This PR improves the heuristic used to select patterns for local
`forall` expressions occurring in the goal being solved by `grind`. It
now considers all singleton patterns in addition to the selected
multi-patterns. Example:
```lean
example (p : Nat → Prop) (h₁ : x < n) (h₂ : ¬ p x) : ∃ i, i < n ∧ ¬ p i := by
grind
```
