pub fn flush(&self, builder: &mut SsTableBuilder) -> Result<()> {
        for entry in self.map.iter() {
            builder.add(KeySlice::from_slice(&entry.key()[..]), &entry.value()[..]);
        }
        Ok(())
    }
Flush the mem-table to SSTable. Implement in week 1 day 6.
mini-lsm/src/mem_table.rs