Macro getter 

macro_rules! getter {
    ($field:ident -> $typ:ty) => { ... };
    (&$field:ident -> $typ:ty) => { ... };
    ($getter_name:ident -> unwrap_or $field:ident $fallback:expr => $typ:ty) => { ... };
    ($getter_name:ident -> $field:ident -> $typ:ty) => { ... };
    ($getter_name:ident -> &$field:ident => $typ:ty) => { ... };
}