feat(code): part 3 iterators

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

mini-lsm-starter/src/iterators/two_merge_iterator.rs

@@ -0,0 +1,38 @@
+#![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 anyhow::Result;
+
+use super::impls::StorageIterator;
+
+/// Merges two iterators of different types into one. If the two iterators have the same key, only
+/// produce the key once and prefer the entry from A.
+pub struct TwoMergeIterator<A: StorageIterator, B: StorageIterator> {
+    a: A,
+    b: B,
+    // Add fields as need
+}
+
+impl<A: StorageIterator, B: StorageIterator> TwoMergeIterator<A, B> {
+    pub fn create(a: A, b: B) -> Result<Self> {
+        unimplemented!()
+    }
+}
+
+impl<A: StorageIterator, B: StorageIterator> StorageIterator for TwoMergeIterator<A, B> {
+    fn key(&self) -> &[u8] {
+        unimplemented!()
+    }
+
+    fn value(&self) -> &[u8] {
+        unimplemented!()
+    }
+
+    fn is_valid(&self) -> bool {
+        unimplemented!()
+    }
+
+    fn next(&mut self) -> Result<()> {
+        unimplemented!()
+    }
+}