feat: model-based theory combination for `ToInt` types

This PR implements model-based theory combination for types `A` which
implement the `ToInt` interface. Examples:
```lean
example {C : Type} (h : Fin 4 → C) (x : Fin 4)
    : 3 ≤ x → x ≤ 3 → h x = h (-1) := by
  grind

example {C : Type} (h : UInt8 → C) (x y z w : UInt8)
    : y + 1 + w ≤ x + w → x + w ≤ z → z ≤ y + w + 1 → h (x + w) = h (y + w + 1) := by
  grind

example {C : Type} (h : Fin 8 → C) (x y w r : Fin 8)
    : y + 1 + w ≤ r → r ≤ y + w + x → x = 1 → h r = h (y + w + 1) := by
  grind
```