What does account code use for? No idea why are 0xff (INVALID) and 0x00 (STOP)...
https://github.com/paritytech/frontier/blob/08dfe8986e3f92ce213244c438eb9178653fce59/primitives/evm/src/lib.rs#L87 https://github.com/paritytech/frontier/blob/08dfe8986e3f92ce213244c438eb9178653fce59/frame/evm/src/tests.rs#L45 https://github.com/paritytech/frontier/blob/08dfe8986e3f92ce213244c438eb9178653fce59/frame/evm/src/tests.rs#L56
