Merge #635

635: Use an unstable algorithm for `grenad::Sorter` when possible r=Kerollmops a=loiclec

# Pull Request
## What does this PR do?

Use an unstable algorithm to sort the internal vector used by `grenad::Sorter` whenever possible to speed up indexing.

In practice, every time the merge function creates a `RoaringBitmap`, we use an unstable sort. For every other merge function, such as `keep_first`, `keep_last`, etc., a stable sort is used.


Co-authored-by: Loïc Lecrenier <loic@meilisearch.com>

milli/src/update/index_documents/mod.rs

@@ -1488,6 +1488,7 @@ mod tests {
         assert_eq!(count, 4);
     }
 
+    #[cfg(feature = "default")]
     #[test]
     fn test_meilisearch_1714() {
         let index = TempIndex::new();