feat: assert_last_event macro

[Daanvdplas]

The following should be improved:

assert_eq!(
		last_contract_event(&session).unwrap(),
		Created {
			id: TOKEN,
			creator: account_id_from_slice(&contract),
			admin: account_id_from_slice(&contract),
		}
		.encode()
		.as_slice()
	);