Changes

Summary

1. tuned whitespace (amending a4bffc0de967);
2. clarified caching;
3. persistent hash code: much faster caching;
4. clarified signature: absorb XZ.Cache into XML.Cache;
5. support more direct hash-consing via XML.Cache;
6. tuned;
7. tuned signature;
8. clarified boundary case;
9. more uniform default --- hardly relevant in practice;
10. proper ssh_port (amending ffd8283b7be0);
11. clarified signature --- internal Cache.none;
12. tuned comments;