improved architecture for performant subfolder support

[kevgo]

Old:

• Tikibase contains a Vec<Document>

New:

• Tikibase contains a root: Directory
• a Directory contains:
  • a configuration: Configuration derived from the tikibase.json file in that directory
  • a Vec<Document> containing the documents directly in this directory
  • a Vec<Directory> containing the subfolders
• a Directory provides:
  • iterator over all documents in it together with a link to the configuration instance that applies to this document

Benefits:

• provides direct access to the containing directory
• makes relative path operations easier