Changes

• Batch requests for transaction receipts.
• Make bigInt.toHex() format numbers as big-endian, with leading zeros trimmed.
• Implement env.abort to improve logging of AssemblyScript exceptions.
• Add GraphQL __typename support.
• Optionally, write subgraph logs to Elasticsearch.
• Add --ethereum-polling-interval / ETHEREUM_POLLING_INTERVAL option.
• Bring the full test suite back (it had been partially disabled due to massive code changes).
• Documentation improvements (getting started).
• Fix casting bug when querying boolean entity fields.
• Add BigInt math (+, -, *, / and %).
• Add more data to Ethereum events, blocks and transactions passed to mappings.