DAL: no rewards for undetected traps

What

This MR makes it so a delegate that has been denounced for not detecting a trap during a cycle does not receive the corresponding rewards for that cycle.

The MR also adds a "denounced" field in the output of the DAL participation RPC.

Why

This allows to test DAL accusations before implementing slashing.

It can be easily reverted when slashing is finalized.

How

We change the storage format of Storage.Dal_already_denounced: instead of a set of tuples (level, slot_index, delegate), we store a map from delegates to (level, slot_index) pairs. We do not expect the length of this list to be big. (this was the initial version, up to and including V3)

We add a new storage Dal_denounced_delegates storing the set of denounced delegates during a cycle (from V4 onwards).