Typescript: Clone maintains `readonly` property status#30

An object (or object tree) to be cloned may have some properties marked as `readonly` to prevent accidental modification in a (shared) object. In the clone, the properties should be modifiable in this use case. I also believe this is the main use case for `readonly`, so changing the return type should not introduce new problems. (Alternatively, two different signatures could be provided; but I think this will needlessly complicate things.)