contracts: New contract events + unconfusions

[Robbepop]

Improves contracts PALLET by

• adds two new events: Restored and Evicted
  • Event::Evicted(AccountId, bool)
    1. contract: AccountID: The account ID of the evicted contract.
    2. tombstone: bool: True if the evicted contract left behind a tombstone.
  • Event::Restored(AccountId, AccountId, Hash, Balance, bool)
    1. donor: AccountId: The Account ID of the restoring contract
    2. dest: AccountId: The account ID of the restored contract
    3. code_hash: Hash: The code hash of the restored contract
    4. rent_allowance: Balance: The initial rent allowance of the restored contract
    5. success: bool: If restoration was successful
• rename Event::Contract to Event::ContractExecution so that it is clear that the event was triggered by a contract execution
• improving error message that have previously be shown to confuse users ("during execution")

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[thiolliere]

I think it miss some evicted event, otherwise looks good

[pepyakin]

Looks good!

Would be great if some tests for these events were provided!

[thiolliere]

Great :-)