Proto: split `delegate_storage.ml`

While trying to understand the delegation and staking code, I found the file delegate_storage.ml a bit harsh to grasp. It is a huge file: ~1000 lines. I tried to split the file in multiple files, with separate concerns to guide my understanding. Just like there is already smaller files like delegate_activation_storage.ml responsible for only one or two keys in the store (for instance delegate_activation_storage.ml manages Storage.Contract.Delegate_last_cycle_before_deactivation and Storage.Contract.Inactive_delegate).

Is there any interest for such a refactoring?

Here is the proposed split:

• delegate_storage.ml
  • Storage.Delegates
  • Storage.Contract.Frozen_deposits_limit
• delegate_missed_endorsements_storage.ml
  • Storage.Contract.Missed_endorsement
• delegate_slashed_deposits_storage.ml
  • Storage.Slashed_deposits
• delegate_sampler.ml
  • Storage.Delegate_sampler_state
• delegate_cycles.ml
  • cycle_end and init_first_cycles

The MR also contains eight related minor refactoring, which simplify some interfaces:

• Proto: move set_active into Stake_storage

  It merges Delegate_table_storage.set_active into Stake_storage.set_active. The function in Delegate_table_storage, do not require access to the part of Storage from which Delegate_table_storage is responsible. Merging the two function simplify the interfaces and avoid having three distinct set_active functions.

• Proto: Rename Delegate.check_delegate into Delegate.check

  Just a renaming.

• Proto: simplify Delegate.registered

  The function Contract_delegate_storage.registered is based on invariant ensured by Delegate_table_storage (i.e. a delegate is self delegated), better to move the function. Then, we simplify the function implementation by requiring only one read access into the context.

• Proto: remove freeze_deposits_do_not_call_except_for_migration

  This function is only called once in the whole protocol code. I suggest to inline it for avoiding incorrect usage. Besides, the name is a bit misleading, the function is only used while migrating from genesis, which is more a chain initialization that a protocol migration.

• Proto: merge Seed.cycle_end into Delegate.cycle_end

  It is to enforce a little bit more the following comment in seed_storage.ml: (* NB: the clearing of past seeds is done elsewhere by the caller *).

• Proto: introduce Alpha_context.Contract.Delegate

  Move the functions Alpha_context.Delegate.{init,set,find} into Alpha_context.Contract.Delegate. These functions basically handle the field 'delegate' of a contract. They do not handle storage related to a given delegate, like other function in Alpha_context.Delegate.

• Prote: rename Delegate.delegate_participation_info into Delegate.participation_info

  Just a renaming.

• Proto/Delegate: remove dead code

  Remove the duplicate function Stake.delegate_pubkey (also available as Delegate.pubkey)