chore: Little clean ups of meilidb-core

meilidb-core/src/reordered_attrs.rs

@@ -5,6 +5,10 @@ pub struct ReorderedAttrs {
 }
 
 impl ReorderedAttrs {
+    pub fn new() -> ReorderedAttrs {
+        ReorderedAttrs { count: 0, reorders: Vec::new() }
+    }
+
     pub fn insert_attribute(&mut self, attribute: u16) {
         self.reorders.resize(attribute as usize + 1, None);
         self.reorders[attribute as usize] = Some(self.count as u16);