Add utility library for easier environment and contract interaction

[y-pakorn]

Will close #10

• Support makerdao's multicall. (https://github.com/makerdao/multicall)
• Add multicall ability to existing ERC20 contract.
• Add basic chain information as enum and extensions.
• Support UniswapV2 (or Uniswap clone such as PancakeSwap) basic interaction and utility such as route calculation. (Based on Uniswap SDK)
• Add Ethereum unit conversion ability using decimal package.
• Complete code documentation.
• Add ERC1155 Contract
• Support automatic JS package import through Dart code by dart:html. (No need for usual index.html editing)