Macro backstage::builder

macro_rules! builder {
    ( $(#[derive($($der:tt),*)])?  $struct:ident {$( $field:ident: $type:tt$(<$($i:tt),*>)? ),*} ) => { ... };
    ($(#[derive($($der:tt),*)])? $struct:ident$(<$($extra:tt),*>)? {$( $field:ident: $type:tt$(<$($i:tt),*>)? ),*}) => { ... };
}

Create the builder type for an actor, which can be used to construct an actor by parts