Derive Clone for Package #64
Description
Consider a case where a template XML is used to produce several other XMLs with different modifications; it seems that at this moment this requires the template to be parsed as many times as the number of desired target XMLs, because Package doesn't implement Clone and cloneing a Document merely copies the references to the parent Package, which doesn't provide a fresh copy of the parsed structure. Having to parse the same file several times is pretty inefficient - cloning Packages would be much faster.
I tried to #[derive(Clone)] for Package myself, but I encountered an issue: typed_arena::Arena isn't Cloneable either. Is there any way around this?