Gas Optimization: No need to use SafeMath everywhere

# Handle

gzeon


# Vulnerability details

## Impact
For example, the following rounding operation is safe

https://github.com/code-423n4/2022-01-openleverage/blob/501e8f5c7ebaf1242572712626a77a3d65bdd3ad/openleverage-contracts/contracts/XOLE.sol#L249
```
        uint256 unlock_time = _unlock_time.div(WEEK).mul(WEEK);
```
to
```
        uint256 unlock_time = (_unlock_time/WEEK)*WEEK;
```

