add key abstraction and prepare for MVCC (#28)

* add key abstraction and prepare for MVCC

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

* a little bit type exercise

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

* refactor tests

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

* fix clippy warnings

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

* refactor starter code

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

* final touch docs

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

---------

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

mini-lsm-starter/src/iterators.rs

@@ -3,11 +3,15 @@ pub mod merge_iterator;
 pub mod two_merge_iterator;
 
 pub trait StorageIterator {
+    type KeyType<'a>: PartialEq + Eq + PartialOrd + Ord
+    where
+        Self: 'a;
+
     /// Get the current value.
     fn value(&self) -> &[u8];
 
     /// Get the current key.
-    fn key(&self) -> &[u8];
+    fn key(&self) -> Self::KeyType<'_>;
 
     /// Check if the current iterator is valid.
     fn is_valid(&self) -> bool;