Merge #402

402: Optimize document transform r=MarinPostma a=MarinPostma

This pr optimizes the transform of documents additions in the obkv format. Instead on accepting any serializable objects, we instead treat json and CSV specifically:
- For json, we build a serde `Visitor`, that transform the json straight into obkv without intermediate representation.
- For csv, we directly write the lines in the obkv, applying other optimization as well.

Co-authored-by: marin postma <postma.marin@protonmail.com>

milli/src/search/distinct/mod.rs

@@ -68,7 +68,9 @@ mod test {
                 "txts": sample_txts[..(rng.gen_range(0..3))],
                 "cat-ints": sample_ints[..(rng.gen_range(0..3))],
             });
-            builder.add_documents(doc).unwrap();
+
+            let doc = Cursor::new(serde_json::to_vec(&doc).unwrap());
+            builder.extend_from_json(doc).unwrap();
         }
 
         builder.finish().unwrap();