Revisit nft_resolve_transfer args

[mikedotexe]

According to the standard:
https://nomicon.io/Standards/NonFungibleToken/Core.html#nft-interface

nft_resolve_transfer has an argument approved_account_ids that is supposed to be string[] but I see there's a HashMap here. There's also a place where that field calls approvals, which should be fixed.

It seems like we don't need to pass the entire HashMap and can stick to the standard and just send a Vec instead.

This is one of those tickets where I noticed it while I'm in the middle of something, so don't have more info other than this at the moment.

[austinabell]

What we probably want here is a Set (probably BTreeSet) for this. A set will serialize/deserialize as an array but comes with the property that we don't have to check for duplicates manually and can still do the lookup more efficiently than iterating through.

I had a quick look and still don't understand the use of storing an integer value for each one. I'll dig into this more tomorrow. (seems like approval ids are currently required to be unique per token, not quite sure why though)

[austinabell]

dug slightly more into this, but it's clear there needs to be discussion around this because I need more context around what the flow is intended to be, and the fact that this is quite a breaking change so if the API is changed we need to be careful.

[frol]

Resolved in #1019