Uniques: Allowing multiple approvals

[Szegoo]

Solves the new approvals structure task from #11783

In short, this PR allows the item owner to make approvals to multiple accounts.

Note: I took a bit of a different approach than proposed in #11791. I believe that this is a bit simpler than storing a BoundedBTreeMap for approvals.

[jsidorenko]

Hi @Szegoo , thanks for your contribution!
I'm thinking it's better to keep the existing Uniques pallet as feature-complete and start pushing all the new changes into V2.
I've created a new branch for this js/uniques-v2-main-branch, this is a new pallet for all V2 works.
It would be good if you could apply your changes on top of that pallet and point your PR to my branch.

[jsidorenko]

The reason I wanted to use BoundedBTreeMap is because of the new structure of approvals I'm proposing: I think each approval should have an optional deadline (the block number) set.
Pls check my prev implementation for reference:
707d69b
a705c22

[Szegoo]

@jsidorenko What would be the benefit of creating a BoundedBTreeMap, cause we could also probably add deadline like this:

/// The approved transferrer of this item, if one is set.
pub(super) approved:
	BoundedVec<(AccountId, Option<<T as frame_system::Config>::BlockNumber>), ApprovalsLimit>,

I am going to use BoundedBTreeMap if you still think we should use that, I am just interested if there is a reason behind this decision.

[jsidorenko]

BoundedBTreeMap simplifies the management of approvals:

1. when you need to find an approval, you will simply call item.approvals.get(&sender) instead of item.approvals.get_key_value(&sender)
2. once you insert, you don't need to check if approval for the same account exists
3. when you remove, you will simply call item.approvals.remove(&delegate) instead of if let Some(pos) = item.approvals.iter().position(|ap| delegate == ap.who) { item.approvals.remove(pos); }

[jsidorenko]

Try to run rustup update and then: cargo +nightly fmt
The latest fmt shouldn't change the syntax that way

[Szegoo]

@jsidorenko I opened a new PR with these changes to the js/uniques-v2-main-branch, Should I close this?