Implementation of a contract to exchange ERC20 tokens from different networks

[MichaelKim20]

1. The code of the smart contract became too big to compile, so I coded the necessary error message.
2. I added a tokenId to distinguish tokens.
   The tokenId is calculated as the address of the bridge, the name of the token, and the token's symbol.

bytes32 tokenId = sha256(abi.encodePacked(address(this), token.name(), token.symbol()));

3. Fees for transactions are collected as native tokens.

The rest are the same as BOAToken Bridge.

Related to #31

[Kang-Simon]

LGTM