Port messages API for new Virtual Machine

[mpapierski]

This PR introduces the ability for new VM smart contracts to emit events as according to the specification described in CEP88 casper-network/ceps#88

The only difference between the implementation in 2.0 in VM1 runtime and the implementation in 2.0 is that under VM2 runtime, the topic is registered lazily in the global state upon first emitting of given message. This is a quality-of-life improvement that also simplifies the code and has potential gas savings (i.e., you don't need to register a topic upfront that may never be emitted). Another difference is that VM2 runtime only allows "Bytes" variant of a payload (compared to String/Bytes option in VM1 runtime).

Usage

use casper_sdk::prelude::*;

#[casper(message)]
pub struct Transfer {
    pub from: Option<Entity>,
    pub to: Entity,
    pub amount: u64,
}

Contract code:

    #[casper(revert_on_error)]
    fn transfer(&mut self, recipient: Entity, amount: u64) -> Result<(), Cep18Error> {
        let sender = casper::get_caller();
        if sender == recipient {
            return Err(Cep18Error::CannotTargetSelfUser);
        }
        self.state_mut()
            .transfer_balance(&sender, &recipient, amount)?;

        casper::emit(Transfer {
            from: Some(sender),
            to: recipient,
            amount,
        })
        .expect("failed to emit message");

        Ok(())
    }

The schema obtained from the contract code contains details regarding the decoding of given message:

{
  ...,
  "definitions": {
    "vm2_cep18::messages::Transfer": {
      "type": "Struct",
      "items": [
        {
          "name": "from",
          "decl": "Option<Entity>"
        },
        {
          "name": "to",
          "decl": "Entity"
        },
        {
          "name": "amount",
          "decl": "U64"
        }
      ]
    },
  "messages": [
    {
      "name": "Transfer",
      "decl": "vm2_cep18::messages::Transfer"
    },
    {
      "name": "Approve",
      "decl": "vm2_cep18::messages::Approve"
    }
  ]

Closes #4993

[igor-casper]

Incomplete comment