message: Expose memory usage in SingleSegmentIndexWriter
diff: @@ -21,6 +21,10 @@ impl SingleSegmentIndexWriter {
  })
  }
  + pub fn mem_usage(&self) -> usize {
  + self.segment_writer.mem_usage()
  + }
  +
  pub fn add_document(&mut self, document: Document) -> crate::Result<()> {
  let opstamp = self.opstamp;
  self.opstamp += 1;
  