Alexandria storage backend

Up to this point alexandria has only existed in memory. This must change before we can actually ship out user clients.

There are a lot of strategies that we could take in Alexandria, but this first pass should focus on simplicity. We can always make things more complicated later.

Also: we currently don't support external binary payloads, which makes things easier as well. Following is an outline of what needs to be implemented for basic persistence.

Outline

• Implement Library::load() to reload an existing session
• Fix path scaffolding in Builder and Dirs
• Implement basic synchronisation engine
  • Sync delta objects
  • Sync metadata store (UserTable, TagCache)

General improvements

• Add Close trait with fn close(&mut self, key: Arc<KeyPair>)
  • Implement trait for any type containing Encrypted<_> types