[컨트랙트] Factory 개선(ref : Uniswap V2)

- *createPair : create token-token pair pool + OPCODE create2 with Pair function(creationCode, initialize)
- the reason of using 'create2' is directly getting the created pair contract address with createPair function

- getter
- 1. getPair : getting token-token pair pool's contract address
- 2. allPairs : getting contract address for token address' pool address
- 3. allPairsLength : getting length of total pool's

- getter/setter
- 1. FeeTo : pool user, transaction fee spender's contract or wallet address
- 2. FeeToSetter : owner(literally FeeTo setter)

[github/UniswapV2Factory.sol](https://github.com/Uniswap/v2-core/blob/master/contracts/UniswapV2Factory.sol)