add compaction skeleton (#16)

* add compaction skeleton

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

* remove tombstone when compact to bottom-most level

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

* new plan

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

---------

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

rustfmt.toml

@@ -0,0 +1,12 @@
+comment_width = 120
+format_code_in_doc_comments = true
+format_macro_bodies = true
+format_macro_matchers = true
+normalize_comments = true
+normalize_doc_attributes = true
+imports_granularity = "Module"
+group_imports = "StdExternalCrate"
+reorder_impl_items = true
+reorder_imports = true
+tab_spaces = 4
+wrap_comments = true