Gas Optimizations

[code423n4]

##GAS

Title: Storage slot packing for gas optimization

https://github.com/code-423n4/2022-05-rubicon/blob/main/contracts/RubiconMarket.sol#L191

The declaration order of state variables affects storage slot packing and gas impact from reads/writes of shared slots. By declaring locked var (bool) next to feeTo (address) can save 1 slot.
Change to:

    uint256 public last_offer_id;

    /// @dev The mapping that makes up the core orderbook of the exchange
    mapping(uint256 => OfferInfo) public offers;

    /// @dev This parameter is in basis points
    uint256 internal feeBPS;

    /// @dev This parameter provides the address to which fees are sent
    address internal feeTo;

    bool locked; //@audit-info: Place here

Title: Using storage instead of memory for struct

https://github.com/code-423n4/2022-05-rubicon/blob/main/contracts/RubiconMarket.sol#L279
https://github.com/code-423n4/2022-05-rubicon/blob/main/contracts/RubiconMarket.sol#L250

By reading directly to storage instead of caching in memory can save execution gas
Change to:

        OfferInfo storage _offer = offers[id];

Title: Using != instead >

https://github.com/code-423n4/2022-05-rubicon/blob/main/contracts/RubiconMarket.sol#L233
https://github.com/code-423n4/2022-05-rubicon/blob/main/contracts/RubiconMarket.sol#L400

Using != operator is more efficient than > for validating that var is not zero