feat(code): finish part 3

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

mini-lsm-starter/src/lsm_iterator.rs

@@ -1 +1,54 @@
+#![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 crate::iterators::impls::StorageIterator;
+use anyhow::Result;
+
 pub struct LsmIterator {}
+
+impl StorageIterator for LsmIterator {
+    fn is_valid(&self) -> bool {
+        unimplemented!()
+    }
+
+    fn key(&self) -> &[u8] {
+        unimplemented!()
+    }
+
+    fn value(&self) -> &[u8] {
+        unimplemented!()
+    }
+
+    fn next(&mut self) -> Result<()> {
+        unimplemented!()
+    }
+}
+
+/// A wrapper around existing iterator, will prevent users from calling `next` when the iterator is invalid.
+pub struct FusedIterator<I: StorageIterator> {
+    iter: I,
+}
+
+impl<I: StorageIterator> FusedIterator<I> {
+    pub fn new(iter: I) -> Self {
+        Self { iter }
+    }
+}
+
+impl<I: StorageIterator> StorageIterator for FusedIterator<I> {
+    fn is_valid(&self) -> bool {
+        unimplemented!()
+    }
+
+    fn key(&self) -> &[u8] {
+        unimplemented!()
+    }
+
+    fn value(&self) -> &[u8] {
+        unimplemented!()
+    }
+
+    fn next(&mut self) -> Result<()> {
+        unimplemented!()
+    }
+}