Add the TopDocs::order_by method.

Uses:
* a `TopOrderable` trait which can be derived for tuples
* a `TopOrderableCollector` to collect for it.
* a `TopNCompare` trait which can be used together with a
  `LazyTopNComputer` to lazily fetch columns during TopN.

Note that this does not use the `CustomScorer` API because:
1. `TopNComputer` does not allow for lazily fetching additional fields
   for the comparison tuple, which is important when tiebreakers are
   only rarely actually coming into play in the comparison, and most
   values are being eliminated by earlier columns.
2. `CustomScoreTopCollector` does not allow segments to Top-N a different type
   than their final output type, which is essential for ordering by `String`s.
3. In order to include scores as one of the ordering columns, we need to be able
   to optionally enable scores.
4. The `CustomScoreTopCollector::merge_fruits` function needs to operate over a
   wrapper type in order to apply different ordering globally than per-segment.

Author: stuhood

Cargo.toml

@@ -56,6 +56,7 @@ itertools = "0.14.0"
 measure_time = "0.9.0"
 arc-swap = "1.5.0"
 bon = "3.3.1"
+paste = "1.0.11"
 
 columnar = { version = "0.5", path = "./columnar", package = "tantivy-columnar" }
 sstable = { version = "0.5", path = "./sstable", package = "tantivy-sstable", optional = true }
@@ -82,7 +83,6 @@ pretty_assertions = "1.2.1"
 proptest = "1.0.0"
 test-log = "0.2.10"
 futures = "0.3.21"
-paste = "1.0.11"
 more-asserts = "0.3.1"
 rand_distr = "0.4.3"
 time = { version = "0.3.10", features = ["serde-well-known", "macros"] }

src/collector/mod.rs

@@ -96,6 +96,9 @@ pub use self::multi_collector::{FruitHandle, MultiCollector, MultiFruit};
 
 mod top_collector;
 
+mod top_orderable;
+pub use self::top_orderable::{Feature, FieldFeature, ScoreFeature};
+
 mod top_score_collector;
 pub use self::top_collector::ComparableDoc;
 pub use self::top_score_collector::{TopDocs, TopNComputer};