[fix] debug prints have to go to stderr

src/reader.rs

@@ -49,11 +49,11 @@ impl AheadReader {
     pub fn read_ahead(&mut self, userbuf: &mut [u8]) -> io::Result<ReadInfo> {
         // 1st read
         if self.bufsz == 0 {
-            println!("[reader] first read");
+            eprintln!("[reader] first read");
             return self.first_read(userbuf);
         }
 
-        println!("[reader] normal read");
+        eprintln!("[reader] normal read");
         // normal read (not the 1st one)
         self.normal_read(userbuf)
     }