Macro match_each_map_builder 

macro_rules! match_each_map_builder {
    ($dyn_builder:expr, | $builder:ident | $body:expr) => { ... };
}

Matches a &mut dyn ArrayBuilder against every concrete map builder type.

Binds the downcast builder as $builder and evaluates $body with it, yielding Some($body); yields None when the builder is not a map builder.