refactor: move iterator trait to parent mod

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

mini-lsm-book/src/04-engine.md

@@ -1 +1,25 @@
 # Storage Engine and Block Cache
+
+
+<!-- toc -->
+
+In this part, you will need to modify:
+
+* `src/lsm_iterator.rs`
+* `src/lsm_storage.rs`
+* Other parts that use `SsTable::read_block`
+
+You can use `cargo x copy-test day4` to copy our provided test cases to the starter code directory. After you have
+finished this part, use `cargo x scheck` to check the style and run all test cases. If you want to write your own
+test cases, write a new module `#[cfg(test)] mod user_tests { /* your test cases */ }` in `table.rs`. Remember to remove
+`#![allow(...)]` at the top of the modules you modified so that cargo clippy can actually check the styles.
+
+## Task 1 - Put and Delete
+
+## Task 2 - Get
+
+## Task 3 - Scan
+
+## Task 4 - Sync
+
+## Task 5 - Block Cache

mini-lsm-book/src/SUMMARY.md

@@ -6,8 +6,8 @@
 
 - [Store key-value pairs in little blocks](./01-block.md)
 - [And make them into an SST](./02-sst.md)
-- [Now it's time for merging everything](./03-memtable.md)
-- [Block cache, the good way](./04-engine.md)
+- [Now it's time to merge everything](./03-memtable.md)
+- [The engine starts](./04-engine.md)
 - [Let's do something in the background](./05-compaction.md)
 - [Be careful when the system crashes](./06-recovery.md)
 - [A good bloom filter makes life easier](./07-bloom-filter.md)