Allow account-level operators to set approval for tokens

[alexytsu]

  Re approval semantics, ERC-721 `approve` says:

Throws unless msg.sender is the current NFT owner, or an authorized operator of the current owner.

This suggests that an address that was nominated via SetApprovalForAll can approve other addresses for specific tokens. But it can't call SetApprovalForAll, and neither can an address approved for a single token approve some other address for that token.

So, should we follow the prior art in this case of allowing an account-wide authorised operator to delegate operation of specific tokens?

Originally posted by @anorth in filecoin-project/FIPs#492 (comment)