Convert BMW implementations into publicly-available PruningScorers. (#162)

As a step towards
[this](paradedb/paradedb#5300), do the
following:
- Introduce a `PruningScorer` trait that extends a `Scorer` to support
setting a threshold.
- Adds a `pruning_scorer` method to the `Weight` trait, with a default
implementation returning a `BasicPruningScorer`.
- Moves the logic for all 3 (union, single, and intersection)
block-max-wand variants into `PruningScorer` implementations so that
they can be driven externally.
- Has `BooleanWeight::pruning_scorer` drive the BMW choice.
- Changes the visibility of the BMW scorers and they're inputs to be
available to consumers of the library.

In repo benchmarks show this change to be a strict improvement for BMW
intersection and at most low-single-digit-percentage-points (<4%) worse
for some BMW query shapes when ran on both my Mac and a dedicated Linux
box.

---------

Signed-off-by: RJ Barman <rjhallsted@gmail.com>
Co-authored-by: Stu Hood <stuhood@paradedb.com>

Author: 2 people

src/indexer/delete_queue.rs

@@ -245,7 +245,7 @@ mod tests {
 
     use super::{DeleteOperation, DeleteQueue};
     use crate::index::SegmentReader;
-    use crate::query::{Explanation, Scorer, Weight};
+    use crate::query::{Explanation, PruningScorer, Scorer, Weight};
     use crate::{DocId, Score};
 
     struct DummyWeight;
@@ -254,6 +254,15 @@ mod tests {
             Err(crate::TantivyError::InternalError("dummy impl".to_owned()))
         }
 
+        fn pruning_scorer(
+            &self,
+            _reader: &SegmentReader,
+            _boost: Score,
+            _init_threshold: Score,
+        ) -> crate::Result<Box<dyn PruningScorer>> {
+            Err(crate::TantivyError::InternalError("dummy impl".to_owned()))
+        }
+
         fn explain(&self, _reader: &SegmentReader, _doc: DocId) -> crate::Result<Explanation> {
             Err(crate::TantivyError::InternalError("dummy impl".to_owned()))
         }