Add support for automerge-like undo/redo

[bobby]

Ideas:

• Annotate each op with patch-id, which is the first lamport timestamp of a given "transaction"/swap!/reset! patch
• Annotate each op with an undoable? boolean, indicating whether it can be undone
• Undos can be done only by the same actor as in the patch-id, and only if undoable? is true
• TODO: figure out mechanics of undo/redo:
  • impact on opset (e.g. appending single-op, non-undoable patches indicating undo/redo of a particular patch?)
  • impact on interpretation (e.g. add an initial pass over opset to build list of no-apply patches to use during interpretation?)