fix nftsByOwner getter

[ilionic]

After merging #31 nftsByOwner requires rework. Currently not reflecting correctly

[HashWarlock]

Currently NftsByOwner is a StorageMap with a key of AccountId mapped to Vec<CollectionId, NftId>. We could use a StorageNMap similar to how uniques pallet handles their assets owned by an account:
Account

#[pallet::storage]
/// The assets held by any given account; set out this way so that assets owned by a single
/// account can be enumerated.
pub(super) type Account<T: Config<I>, I: 'static = ()> = StorageNMap<
	_,
	(
		NMapKey<Blake2_128Concat, T::AccountId>, // owner
		NMapKey<Blake2_128Concat, T::ClassId>,
		NMapKey<Blake2_128Concat, T::InstanceId>,
	),
	(),
	OptionQuery,
>;

remove & insert from the StorageNMap

Account::<T, I>::remove((&details.owner, &class, &instance));
Account::<T, I>::insert((&dest, &class, &instance), ());

owned

/// Returns an iterator of the asset instances of all classes owned by `who`.
///
/// NOTE: iterating this list invokes a storage read per item.
fn owned(who: &T::AccountId) -> Box<dyn Iterator<Item = (Self::ClassId, Self::InstanceId)>> {
	Box::new(Account::<T, I>::iter_key_prefix((who,)))
}

owned_in_class

/// Returns an iterator of the asset instances of `class` owned by `who`.
///
/// NOTE: iterating this list invokes a storage read per item.
fn owned_in_class(
	class: &Self::ClassId,
	who: &T::AccountId,
) -> Box<dyn Iterator<Item = Self::InstanceId>> {
	Box::new(Account::<T, I>::iter_key_prefix((who, class)))
}

[h4x3rotab]

Given there's already a reverse index available in uniques, I suggest to just keep using the upstream's. Very clever design: there's no Vec<>, and therefore the performance problem was walked around.

[bmacer]

@h4x3rotab what is this reverse indexing available in uniques?

[h4x3rotab]

@h4x3rotab what is this reverse indexing available in uniques?

This one. You can go through all the NFTs root-owned by its owner:

https://github.com/paritytech/substrate/blob/57bf92e42c2ca7f3444567b5ce50d2744fe90b4d/frame/uniques/src/lib.rs#L136

[bmacer]

NFtsByOwner removed in #91