Add Active Record based setup.

[kaspth]

Swaps our tests over to transactional tests and removes the TODO for that.

We'll also start using Active Record based models as the default setup for now, memory.register SomeModel is still available for memory based ones.

I've also added a records.preregister setup so users shouldn't need to call register manually for all their Active Record based models. I still don't know where to automatically hook in to Active Record's boot and call it, or whether we can do it piecemeal for partial seeding. But this is a start!

[kaspth]

I looked into how Active Record dumps a schema into db/schema.rb to figure out the ActiveRecord::Base.connection.tables by the way. It's here: https://github.com/rails/rails/blob/main/activerecord/lib/active_record/schema_dumper.rb#L122