feat(book): finish overview

Signed-off-by: Alex Chi <iskyzh@gmail.com>

mini-lsm-starter/src/block/iterator.rs

@@ -0,0 +1,62 @@
+#![allow(unused_variables)] // TODO(you): remove this lint after implementing this mod
+#![allow(dead_code)] // TODO(you): remove this lint after implementing this mod
+
+use std::sync::Arc;
+
+use super::Block;
+
+pub struct BlockIterator {}
+
+impl BlockIterator {
+    fn new(block: Arc<Block>) -> Self {
+        unimplemented!()
+    }
+
+    pub fn create_and_seek_to_first(block: Arc<Block>) -> Self {
+        unimplemented!()
+    }
+
+    pub fn create_and_seek_to_key(block: Arc<Block>, key: &[u8]) -> Self {
+        unimplemented!()
+    }
+
+    pub fn key(&self) -> &[u8] {
+        unimplemented!()
+    }
+
+    pub fn value(&self) -> &[u8] {
+        unimplemented!()
+    }
+
+    pub fn is_valid(&self) -> bool {
+        unimplemented!()
+    }
+
+    pub fn seek_to_first(&mut self) {
+        unimplemented!()
+    }
+
+    pub fn seek_to_last(&mut self) {
+        unimplemented!()
+    }
+
+    pub fn len(&self) -> usize {
+        unimplemented!()
+    }
+
+    pub fn is_empty(&self) -> bool {
+        unimplemented!()
+    }
+
+    pub fn seek_to(&mut self, idx: usize) {
+        unimplemented!()
+    }
+
+    pub fn next(&mut self) {
+        unimplemented!()
+    }
+
+    pub fn seek_to_key(&mut self, key: &[u8]) {
+        unimplemented!()
+    }
+}