Function map_insert 

pub fn map_insert(
    map: &mut CandelaMap,
    key: Data,
    value: Data,
    strings: &StringPool,
) -> Option<Data>

Puts value under key, replacing the value of a key with the same text if the map holds one, and answers the value it replaced. Every insert goes through here, so every key a map holds is filed by Data::as_map_key.