Offer/accept functionality

[znebby]

We need a two-step alternative to transfernft which causes the receiver to assume RAM costs.

This is absolutely necessary for dapps and marketplaces. At about 0.5kb per NFT, which has a current cost of about $0.21, many dapps that receive and send a lot of NFTs simply won't be able to afford it, and may be forced to use other standards instead of dgoods.