Mint/Transfer return optimisation

[abright]

Follows on from #103 (comment)

The new model requires querying balance and allowance data separately after every mint or transfer is completed, in order to deal with possible actions taken inside the receiver hook. If state didn't change inside the receiver hook call, we could safely use the values we get from within the mint/transfer implementations.

It should be easy to pass through those values in the intermediate data so they are ready for use, the tricky part here is knowing when the state is 'dirty' and we need to query fresh values. We could save the root CID in there as part of making the hook call and compare it afterwards (in mint_return) to decide but maybe there's a better way to do it?

[abright]

Closing this and continuing on in #139