[CodeArena] - [G-31] : Remove token variable from struct Holding

[maximebrugel]

Issue => code-423n4/2021-11-nested-findings#121

This change allows to save around 1k Gas, this is an important optimization.

However, it will impact the front-end, because the function tokenHoldings will not longer contain the token field.

Here are the different possibilities :

• Add address token or address[] tokens alongside Holding. But not convenient for the front-end, since we are removing the tuple.
• Create a new struct inside NestedRecords (but keep Holding) to match the front-end needs. But will increase the bytecode size.
• Create a batcher smart contract for custom views. But need to store a new address in the front-end.

[maximebrugel]

We could remove the isActive to remove the struct Holding :

struct NftRecord {
    mapping(address => uint256) holdings;
    address[] tokens;
    address reserve;
    uint256 lockTimestamp;
}

Then adapt the view functions.

[maximebrugel]

@adrien-supizet The struct Holding is now removed.