contract_sample

contract_sample contains the following contract code

1. ERC20, a template which issue tokens on ethernum network, and can be deposited to layer2 to trade for zero gas fee
2. ERC721, which mint on ethernum network(layer1), and can be deposited to layer2 to trade for zero gas fee
3. ERC721Mintfor, which can be used mint on layer2 and withdrawal to layer1