Add borrowing

[jerel]

This provides the ability to borrow Dart types from another namespace instead of generating a separate copy into each namespace. Example:

#[async_dart(
  namespace = "orgs",
  borrow = "locations::Location",
  borrow = "accounts::Contact",
  borrow = "accounts::Filter",
  borrow = "accounts::Match",
  borrow = "accounts::Status"
)]
pub async fn get_org_with_borrowed_type(id: data::Filter) -> Result<data::Organization, String> {
  let _ = id;
  Ok(data::Organization {
    id: 1,
    owner: data::Contact::default(),
    location: data::Location {
      polyline_coords: vec![(-104.0185546875, 43.004647127794435)],
    },
  })
}

The borrow format is namespace::Class.