Implement CREATE2 Opcode

[cburgdorf]

What was wrong?

We don't support the CREATE2 Opcode for Constantinople yet.

How was it fixed?

Implemented CREATE2 Opcode based on the existing CREATE opcode. Broken down:

• address format is latest and greatest from last core dev call
• reading items from the stack was abstracted into method get_stack_data to allow overwriting and hence read with/without salt
• generating address was abstracted into method generate_contract_address to allow overwriting and hence implement the different version format

Known issues

• not tested yet

Cute Animal Picture

[cburgdorf]

Note to self: CREATE2 state tests are coming. Should be helpful to validate the implementation (which isn't expected to work yet)

ethereum/tests#486

[pipermerriam]

This might be easier to read (and slightly more cleanly typed as) as:

return force_bytes_to_address(keccak(
    decode_hex('0xff') + address + int_to_big_endian(salt) + keccak(call_data)
))

Also, the decode_hex('0xff') should probably be replaced with a constant CREATE2_PREFIX or just a bytes literal.

[cburgdorf]

Right! Changing it to:

    return force_bytes_to_address(
        keccak(b'\xff' + address + int_to_big_endian(salt) + keccak(call_data))
    )

I'll also change generate_contract_address to use force_bytes_to_address to be aligned with this change.