Eip-3198: basefeeopcode

Add a BASEFEE (0x48) that returns the value of the base fee at the current block.


Compatibility planning:

On blockchains without EIP-1559, I would expect BASEFEE to return 0. I expect most contracts using BASEFEE to do GASPRICE - BASEFEE, and I expect the constraint BASEFEE <= GASPRICE to hold in such case.

An overview of EIP-3198: BASE FEE opcode with @rai.