Parses simple blocks after a block group are silently ignored.

[Matthias-Fauconneau]

Simple blocks after a block group are silently ignored.
Parsing into the existing simple_block would reorder.
This parses all simple blocks into struct BlockGroup to allow any interleaving of simple blocks or block groups inside a cluster while preserving the order.
If that's an acceptable solution, the serialization needs to be updated.

[Luni-4]

@Matthias-Fauconneau What's the status of this PR?

[Matthias-Fauconneau]

I think that's the proper solution.
Deserialization works for me.
You might want to also do the serialisation part though.

[Luni-4]

Could you please open an issue specifying what is left to do and close this PR? In this way I can remember what we need to implement :)

Thank you!