EIP-1344 CHAINID

Documentation for tests/istanbul/eip1344_chainid.

Generate fixtures for these test cases with:

fill -v tests/istanbul/eip1344_chainid

abstract: Tests EIP-1344: ChainID Opcode

Test cases for [EIP-1344: ChainID Opcode](https://eips.ethereum.org/EIPS/eip-1344).