Function written_size 

pub fn written_size<S, R>(comp: &Compound<S>, root_name: &R) -> usize
where
    S: ToModifiedUtf8 + Hash + Ord,
    R: ToModifiedUtf8 + ?Sized,

Returns the number of bytes that will be written when to_binary is called with this compound and root name.

If to_binary results in Ok, the exact number of bytes reported by this function will have been written. If the result is Err, then the reported count will be greater than or equal to the number of bytes that have actually been written.