Uniswap differences

The code is forked from Uniswap V2 and maintains a strong similarity. Apart from renaming variables, the following changes can be observed in the DeltaSwapPair contract when comparing DeltaSwap with Uniswap V2 at commit ee547b17↗:

• The swap fee is adjustable instead of a fixed three base points, as it is obtained by calling the factory.

• The uint was replaced with the more explicit uint256.

• The target Solidity version was bumped, and SafeMath operations were replaced by built-in checked math.