Reference implementation of an ERC-721 NFT adapter.
Becacuse each individual token must be its own ilk (since risk parameters and price feeds are per-ilk), we introduce the concept of kin, or family of _ilk_s. An example of kin is "cryptokitties".
An adapter must be deployed for each kin.
The adapter accepts NFT tokens (obj) with join, and adds
them to a user's CDP.
When the user wants to take the NFTs out of the system, they call exit.