Add method to take clauses out of boolean query

This helps with rewriting, for example to replace the default
score combiner used by boolean queries, insofer it avoids
the need to `box_clone` the sub queries.

Author: Adam Reichold

src/query/boolean_query/boolean_query.rs

@@ -1,3 +1,5 @@
+use std::mem;
+
 use super::boolean_weight::BooleanWeight;
 use crate::query::{EnableScoring, Occur, Query, SumCombiner, TermQuery, Weight};
 use crate::schema::{IndexRecordOption, Term};
@@ -254,8 +256,13 @@ impl BooleanQuery {
     }
 
     /// Deconstructed view of the clauses making up this query.
-    pub fn clauses(&self) -> &[(Occur, Box<dyn Query>)] {
-        &self.subqueries[..]
+    pub fn clauses(&self) -> impl Iterator<Item = &(Occur, Box<dyn Query>)> {
+        self.subqueries.iter()
+    }
+
+    /// Take the clauses making up this query for reuse.
+    pub fn take_clauses(&mut self) -> impl Iterator<Item = (Occur, Box<dyn Query>)> {
+        mem::take(&mut self.subqueries).into_iter()
     }
 }