Provide a way to materialize an OwnedElement from the current value of a previously initialized reader

[dlurton]

Possibly, this is just a single function on StreamReader, like this:

trait StreamReader {
    /// Materializes only the current value from the specified `reader`.  
    pub fn read_element<'a>(&mut self) -> IonResult<Option<OwnedElement>> {
        ...
    }
}

This would ultimately invoke this function to actually perform the materialization.

[zslayton]

Fixed in #485.

[dlurton]

#485 does not actually satisfy this request. The none of the ElementReader functions introduced there read the current value of the StreamReader. The closest one, StreamReader::read_next_element advances to the next element and then reads. I need one that just reads the current element.