Macro def_default

macro_rules! def_default {
    ($({ $field:ident, $type:ty, $default: expr, $($rest:tt)* },)*) => { ... };
    (@ $field:ident, $type:ty, None) => { ... };
    (@ $field:ident, $type:ty, $default: expr) => { ... };
}

Define default value functions for those with Some default values.