This should use `System.arraycopy`.

Inserting a array of items makes sense, but I'm not sure about set. It's kind of like "overwrite", feels a bit strange calling it "set". I think I'd rather call it "copy" and have it work like `System.arraycopy`, with source index, destination index and count. There should be an overload that takes `Array<T>`.

This method needs to be implemented for all Array classes (IntArray, etc).