text: Organize the project:
  - Create blockchain/
  - Move block.js, block.test.js, blockchain.js, blockchain.test.js to blockchain/
  - Rename blockchain.js and blockchain.test.js to index.js and index.test.js
  - In index.test.js, update the blockchain requirement:
    `const Blockchain = require('./index');`
  - Create app/
  - Create app/index.js
  