Add checks on object deserialization

[damip]

• inside block header deserialization add the following checks:

  • check the signature (now we can because we see each object only once)
  • check that (parents.count == 0) == (slot.period == 0)
  • check that if slot.period == 0 we have no endorsements
  • check the signatures of the endorsements
  • check that the endorsements all target the parent block in the same thread (if not genesis)
  • check that the index of every endorsement does not repeat
  • check that the index of every endorsement does not overflow endorsement_count-1

• inside op deserialization:

  • check signature

• inside endorsement deserialization:

  • check signature