Improve type of Const functor

https://github.com/modular-implicits/imp/pull/1#discussion_r1288254855

```diff
- type ('a, 'b) const = Const of 'a
+ type ('a, 'b) const = 'a
```

or

```diff
- type ('a, 'b) const = Const of 'a
+ type ('a, 'b) const = { const: 'a }
```