Support message types that are self referential

[codyoss]

Rust does not like self-referential types, at least without indirection. We will need to Box types that are self referential as such. Example

message Node {
  Node next = 1;
  Node prev = 2;
  string val = 3;
}

pub struct Node {
  pub next: Option<Box<Node>>,
  pub prev: Option<Box<Node>>,
  pub val: String,
}