This PR improves support for structure extensionality in `grind`. It now
uses eta expansion for structures instead of the extensionality theorems
generated by `[ext]`. Examples:
```lean
opaque f (a : Nat) : Nat × Bool
attribute [grind ext] Prod Subtype
example (a b : Nat) : (f a).1 = (f b).1 → (f a).2 = (f b).2 → f a = f b := by
grind
def g (a : Nat) : { x : Nat // x > 1 } :=
⟨a + 2, by grind⟩
example (a b : Nat) : (g a).1 = (g b).1 → g a = g b := by
grind
@[grind ext] structure S where
x : Nat
y : Int
example (x y : S) : x.1 = y.1 → x.2 = y.2 → x = y := by
grind
```
