Add `@warn_unused_result` to `set()`

```
    @warn_unused_result
    func set(newPart: Part) -> Whole {
        return boundLensStorage.lens.set(newPart, boundLensStorage.instance)
    }
```

This would make

```
data.throughLens.name.set("Daniel")
```

result in a warning. Which is good, because the code should probably be something like

```
data = data.throughLens.name.set("Daniel")
```

or similar.
