feat: `ToInt` equality in `grind cutsat` (#9051)

This PR implements support for equalities and disequalities in `grind
cutsat`. We still have to improve the encoding. Examples:
```lean
example (a b c : Fin 11) : a ≤ 2 → b ≤ 3 → c = a + b → c ≤ 5 := by
  grind

example (a : Fin 2) : a ≠ 0 → a ≠ 1 → False := by
  grind
```