fix: better case-split for match-conditions in `grind`

This PR improves the case splitting strategy used in `grind`, and
ensures `grind` also considers simple `match`-conditions for
case-splitting.  Example:

```lean
example (x y : Nat)
    : 0 < match x, y with
          | 0, 0   => 1
          | _, _ => x + y := by -- x or y must be greater than 0
  grind
```