Gas Optimizations

[code423n4]

Title: Use lendTicketContract, borrowTicketContract as immutable variables

Impact

For any variable that is only set once, either use a constant or immutable to save gas. The compiler does not reserve a storage slot for these variables, and every occurrence is replaced by the respective value (Source: https://docs.soliditylang.org/en/v0.6.5/contracts.html#constant-and-immutable-state-variables)

Recommended Mitigation Steps

Remove functions setLendTicketContract() and setBorrowTicketContract(), change variables lendTicketContract, borrowTicketContract as immutable and initialize them in constructor:
https://github.com/code-423n4/2022-04-backed/blob/main/contracts/NFTLoanFacilitator.sol#L275-L293

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Title: Use Solidity v0.8.4 because of Solidity Custom Errors

Impact

Starting from Solidity v0.8.4, there is a convenient and gas-efficient way to explain to users why an operation failed through the use of custom errors. Custom errors decrease both deploy and runtime gas costs. Source: https://blog.soliditylang.org/2021/04/21/custom-errors/

OpenZeppelin is also planing to use custom errors starting with next major release 5.0.
Source release v5.0: OpenZeppelin/openzeppelin-contracts#2961
Source OZ custom error issue: OpenZeppelin/openzeppelin-contracts#2839

Recommended Mitigation Steps

Refactor code and use Solidity custom errors.

[wilsoncusack]

this is not possible because we pass in LoanFacilitator address to the borrow and lend tickets on creation, so we do not have them when loan facilitator is created. Chicken and egg