fix: unsafe casts better explained

[lekhovitsky]

Acknowledges https://github.com/spearbit-audits/review-gearbox/issues/9

[StErMi]

@lekhovitsky part of the unsafe casts / type conversions have been documented via the PR, but there are still some of them that have not been covered by a dev comment.

• AccountFactoryV3.sol#L102, CreditConfiguratorV3.sol#L186, PoolQuotaKeeperV3.sol#L416, PoolV3.sol#L135, PoolV3.sol#L575, PoolV3.sol#L579, PoolV3.sol#L645: uint40(block.timestamp)
• CreditFacadeV3.sol#L343: lossParams.currentCumulativeLoss += uint128(reportedLoss);
• CreditFacadeV3.sol#L868, CreditManagerV3.sol#L417: lastBlockBorrowed = uint64(block.number), currentCreditAccountInfo.lastDebtUpdate = uint64(block.number)
• CreditFacadeV3.sol#L876: totalBorrowedInBlock = uint128(newDebtInCurrentBlock)

@lekhovitsky would you mind providing an official statement to acknowledge the remaining items to be covered?

[lekhovitsky]

@StErMi

• block.number and block.timestamp casts are safe for sufficiently long time not to worry about it, even on chains with sub-second block times;
• Cumulative loss logic is removed in #280, so this line can be ignored (but cast safety follows from the assumption that underlying amounts fit into uint96);
• uint128(newDebtInCurrentBlock) has been documented even before fixes.

[StErMi]

@lekhovitsky PR #280 cannot be considered in the context of the fix review because it is not part of the PR included in the fix review. I must consider it as not existing unfortunately.