refactor: move iterator trait to parent mod

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

mini-lsm/src/iterators.rs

@@ -1,6 +1,19 @@
-pub mod impls;
 pub mod merge_iterator;
 pub mod two_merge_iterator;
 
+pub trait StorageIterator {
+    /// Get the current value.
+    fn value(&self) -> &[u8];
+
+    /// Get the current key.
+    fn key(&self) -> &[u8];
+
+    /// Check if the current iterator is valid.
+    fn is_valid(&self) -> bool;
+
+    /// Move to the next position.
+    fn next(&mut self) -> anyhow::Result<()>;
+}
+
 #[cfg(test)]
 mod tests;