polish: add msig pendingtxn diffing and comp

[frrist]

add diffing logic for multisig actors pending transcription HAMT

[Stebalien]

This works but I'm not sure how useful the diff itself will be in practice. It won't tell you if a transaction has been approved or was canceled.

[frrist]

This works but I'm not sure how useful the diff itself will be in practice. It won't tell you if a transaction has been approved or was canceled.

True. The motivation here is to allow Visor to track changes in the multisigs pending transaction HAMT.

One way to obtain the result of these transactions is by inspecting the return value of Propose, Approve, and Cancel messages sent to a multisig actor.