Macro build_engine

macro_rules! build_engine {
    (
        codec: $codec:expr,
        fitness: $fitness_fn:expr,
        settings: { $( $setting:ident $( : $value:expr )? ),* $(,)? }
    ) => { ... };
    (@apply_setting $builder:ident, $method:ident, $value:expr) => { ... };
    (@apply_setting $builder:ident, $method:ident) => { ... };
}