Add utility library

[y-pakorn]

Ideas, Feedbacks, and/or help on pr are always appreciated.

• 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)
• Support automatic JS package import through Dart code by dart:html. (No need for usual index.html editing) Seems like flutter web doesn't support isolate yet so we can't mark the entry point of the script to be running first, normal DOM script appending doesn't load the script as well. This is impossible for now. Possible now using AMDJS package
• Add Ethereum unit conversion ability using decimal package.

In consideration

• Support ERC1155 and ERC721. Because the different contract has different implementation and metadata, maybe as a base class for to be extended on.

[y-pakorn]

Automatic JS package injection is possible now using AMDJS package, available for usage in pre-release.