fn read_from_dst(&mut self, buf: &mut [u8]) -> usize {
        let available_bytes = self.dste - self.dsts;
        let count = cmp::min(available_bytes, buf.len());
        buf[..count].copy_from_slice(&self.dst[self.dsts..self.dsts + count]);
        self.dsts += count;
        count
    }
Read previously compressed data from `self.dst`, returning the number of
bytes read. If `self.dst` is empty, returns 0.
src/read.rs