IDL Messages: Prepend and skip magic bytes and type description

[nomeata]

No description provided.

[nomeata]

Not super clean yet, but should be good enough to do corresponding things in the JS world in parallel.

[nomeata]

@chenyan-dfinity I think you were not at the meeting before I had to leave. This PR adds the type description to the messages, and skips it upon decoding, not making use of the data. So I think the next step (besides me cleaning up this code) is to extend the JS side to do the same: Write a type description, and skip it upon reading.

Once that is in place, we can – now in parallel! – start making use of the type data, i.e. fail if it does not match the expected type, and also implement the already agreed on parts of subtyping (records, variants, nat <: int).

Incidentially, because we are still working with the old system api where we serialize messages independently, all of this is orthogonal from the discussion in #465 about whether argument lists should stay records or become width-subtyping tuples and we are not blocked on that yet.

[nomeata]

I wouldn’t be ashamed of putting this master, clearly much of it will be re-written as we start making use of the section.

But as before, I’ll wait for corresponding support on the JS side, and I will leave this in “draft” state until that point.

Incidentially, the IDL code is also getting complicated, so we need to think about more rigorous testing, including fuzz testing. Maybe someone who did not write the code should get into a malicious mindset and try hard to break it?

[chenyan-dfinity]

We can merge this and work in parallel now.