feat(lsm): implement a simple trie remove

lsm/include/lsm/bt.h

@@ -20,6 +20,16 @@ lsm_error lsm_bt_init(lsm_bt **ptr);
  */
 void lsm_bt_free(lsm_bt *bt);
 
+/**
+ * Remove the binary tree's entire contents, but keep the struct allocated.
+ */
+void lsm_bt_clear(lsm_bt *bt);
+
+/**
+ * Return the size of the binary tree
+ */
+uint64_t lsm_bt_size(lsm_bt *bt);
+
 /**
  * Search for the data stored behind the given key.
  *