DAL: merge attestations

This MR puts DAL attestation content inside consensus attestations. It does not change the encoding format of the existing consensus attestation operation; instead it adds a new format, which is used when the (OCaml) consensus attestation has some DAL content.

The baker now includes DAL content into consensus attestations, not using DAL attestation operations anymore.

This MR does not however remove the DAL attestation operation; this will be done in a follow-up MR.

Finally, a note on comparison of and conflicts between consensus attestations:

• the weight includes now the number of attested DAL slots, and the more the better; so when two attestations have the same level, round, and (consensus) slot, but different number of DAL slots, the one with more attested slots is better.
• the conflict condition does not change, it is still based on the tuple (slot, level, round).

Given that it's a relative big MR, it's probably better to review it commit by commit.