Change data section index from `0x04` to `0xff`

[pdobacz]

Motivation: data has always have to be the last section, because it makes appending auxdata to it easy.

[chfast]

If data section is mandatory and have 0xff id it works as header terminator so we can drop the 0x00 one.

[pdobacz]

If data section is mandatory and have 0xff id it works as header terminator so we can drop the 0x00 one.

I forgot to post this here. My concern is what if we change our minds to have data section header optional? With the terminator byte that could be backwards compatible change.

This isn't a huge reason not to do this, just some tiny drawback (and optional data section has been considered, now not possible for good reasons, but maybe someday?)

If this argument holds water I'll later transfer it to the relevant EIP-3540 Rationale.

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Beyond that, I would treat it as an additive change wrt the 0x04->0xff change, decoupled

[chfast]

Having the data section optional is fine to me: it brings some consistency to the design: e.g. there are no empty sections.