Issue/471 (#481)

* Closes 471

Removing writing_segments in the segment manager as it is now useless.
Removing the target merged segment id as it is useless as well.

* RAII for tracking which segment is in merge.

Closes #471

* fmt

* Using Inventory::default().

src/tokenizer/lower_caser.rs

@@ -50,7 +50,6 @@ where
                 self.token_mut().text.make_ascii_lowercase();
             } else {
                 to_lowercase_unicode(&mut self.tail.token_mut().text, &mut self.buffer);
-
                 mem::swap(&mut self.tail.token_mut().text, &mut self.buffer);
             }
             true