add simple leveled compaction

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

mini-lsm/src/compact.rs

@@ -1,10 +1,14 @@
 mod leveled;
+mod simple_leveled;
 mod tiered;
 
 use std::sync::Arc;
 
 use anyhow::Result;
 pub use leveled::{LeveledCompactionController, LeveledCompactionTask};
+pub use simple_leveled::{
+    SimpleLeveledCompactionController, SimpleLeveledCompactionOptions, SimpleLeveledCompactionTask,
+};
 pub use tiered::{TieredCompactionController, TieredCompactionOptions, TieredCompactionTask};
 
 use crate::iterators::merge_iterator::MergeIterator;