Message field design

[stepancheg]

Message fields should be stored as:

message_field: Option<Box<MyMessage>>

However, it is not convenient to work with data types like this:

• obtaining a message or default value is inconvenient
• assignment is inconvenient either

Current design for version 3 is to have MesssageField<T> data type:

struct MessageField<M: Message>(pub Option<Box<M>>);

impl<M: Message> MessageField {
  fn get_or_default(&self) -> &M {
    match &self.0 {
      Some(m) => m,
      None => M::default_instance(),
    }
  }
}

message_field: MessageField<MyMessage>,