feat(index): update fields distribution in clear & delete operations

fixes after review

bump the version of the tokenizer

implement a first version of the stop_words

The front must provide a BTreeSet containing the stop words
The stop_words are set at None if an empty Set is provided
add the stop-words in the http-ui interface

Use maplit in the test
and remove all the useless drop(rtxn) at the end of all tests

Integrate the stop_words in the querytree

remove the stop_words from the querytree except if it was a prefix or a typo

more fixes after review

milli/src/external_documents_ids.rs

@@ -19,6 +19,11 @@ impl<'a> ExternalDocumentsIds<'a> {
         }
     }
 
+    /// Returns `true` if hard and soft external documents lists are empty.
+    pub fn is_empty(&self) -> bool {
+        self.hard.is_empty() && self.soft.is_empty()
+    }
+
     pub fn get<A: AsRef<[u8]>>(&self, external_id: A) -> Option<u32> {
         let external_id = external_id.as_ref();
         match self.soft.get(external_id).or_else(|| self.hard.get(external_id)) {