#442 sometimes produced unsound type inference, such as in case where
properties with the same name might be parsed into different values.

```typescript
const P = Object({ n: String.withParser(() => 1 as const) })
const N = Object({ n: String.withParser(parseInt) })
const PN = P.and(N)
assertEquals(PN.parse({ n: "42" }).n, 1)
// `n` is infered as `1`, but `undefined` actually.
```

I can't imagine any viable solution to this, so reverting.