Adding a method which applies cleaners to selected fields

Haha, poor Jithin! He originally had it as a method on the object, and I told him to move it here. :)

I am not sure what the advantage of having it on the object is compared to the instance:
```
Clean.applyTo(_ == Fields.Foo, Clean.default)
// vs
Clean.default.applyTo(_ == Fields.Foo)
```