Introduce a new search module, eventually meant to replace the old one

The code here does not compile, because I am merely splitting one giant
commit into smaller ones where each commit explains a single file.

milli/src/search/new/mod.rs

@@ -0,0 +1,55 @@
+pub mod db_cache;
+pub mod graph_based_ranking_rule;
+pub mod query_graph;
+pub mod query_term;
+pub mod ranking_rule_graph;
+pub mod ranking_rules;
+pub mod resolve_query_graph;
+pub mod sort;
+pub mod words;
+
+use charabia::Tokenize;
+use heed::RoTxn;
+pub use query_graph::*;
+pub use ranking_rules::*;
+use roaring::RoaringBitmap;
+
+use self::{
+    db_cache::DatabaseCache,
+    query_term::{word_derivations, LocatedQueryTerm},
+};
+use crate::{Index, Result};
+
+pub enum BitmapOrAllRef<'s> {
+    Bitmap(&'s RoaringBitmap),
+    All,
+}
+
+pub fn make_query_graph<'transaction>(
+    index: &Index,
+    txn: &RoTxn,
+    db_cache: &mut DatabaseCache<'transaction>,
+    query: &str,
+) -> Result<QueryGraph> {
+    assert!(!query.is_empty());
+    let fst = index.words_fst(txn).unwrap();
+    let query = LocatedQueryTerm::from_query(query.tokenize(), None, |word, is_prefix| {
+        word_derivations(
+            index,
+            txn,
+            word,
+            if word.len() < 4 {
+                0
+            } else if word.len() < 100 {
+                1
+            } else {
+                2
+            },
+            is_prefix,
+            &fst,
+        )
+    })
+    .unwrap();
+    let graph = QueryGraph::from_query(index, txn, db_cache, query)?;
+    Ok(graph)
+}