loaders: Add getSources() to ContractResult

[shazow]

• Added getSources?: () => Promise<ContractSources>; to ContractResult
• Currently only supported by the Etherscan loader
• ContractSources automatically deserializes the encoded source code structure. This will help once we add support for the Sourcify loader which uses a different format (and will require a separate request for each import).
• ContractResult is not yet accessible from AutoloadResult, so would need to use loaders directly to access it for now. (This will require a bit more plumbing, since autoload uses loadABI rather than loadContract, so we'll need yet another flag to include contract sources.)
• Fixes #111

Usage:

const result = await loader.getContract(address);
const sources = await result.getSources();

for (const s of sources) {
  console.log(s.path, " -> ", s.content.slice(25) + "...");
}

[shazow]

Alright I think this is ready for merge and then probably new release time!