Function merge_map 

pub fn merge_map<'buf, K, V, M: Map<K, V>>(
    this: &mut M,
    stream: &mut InputStream<'buf>,
    kmapper: fn(&mut InputStream<'buf>, &mut K) -> Result<()>,
    vmapper: fn(&mut InputStream<'buf>, &mut V) -> Result<()>,
) -> Result<()>
where
    K: Default,
    V: Default,