Persistence Refactor

[NathanWolf]

This is a catch-all for some short-term work:

• Fix using an object as another object's id- seems to be broken. I have some test code now, so that will help a lot!
• Allow adding or retrieving persisted classed programmatically.
• Allow for adding persisted fields programmatically. This could be used in conjunction with the above to persist a third-party or otherwise unmodifiable class. It could also be used to extend an existing persisted class (though eventually I will traverse up the class hierarchy automatically, looking for persisted parent classes)
• Allow for read-only attributes, such as the id field of BlockVector. This is "read-only" from Persistences' perspective.

[NathanWolf]

Done, son!

Still need to get auto schema migration working and some other stuff I'm going to make separate issues for.