Use-case: replace ipfs object diff with a generalized version that works for generalized IPLD DAGs.

Text syntax:

• Use + for added items.
• Use - for removed items.
• Use ~ for replaced items.
• Syntax: OP PATH VALUE

JSON syntax: Use JSON patch.
Generalized syntax: Basically, JSON patch with extra CBOR types.

Blocker: Pathing through DagPB nodes in go-ipfs isn't correct per the IPLD spec (paths don't reflect the object structure). See ipld/specs#55.