Don't copy around DirtyThreats

The contents of DirtyThreats gets memcpyed twice for each call to do_move.
(Return value optimization doesn't apply to the do_move function itself because
it constructs a std::pair, so it gets copied; and the calls to reset also
require a copy.) This patch inserts the dirty info in place.

Sometimes the caller of do_move ignores the DirtyThreats info, so we pass in
scratch objects. I found that stack-allocating these scratch objects was bad on
Fishtest, so I begrudgingly put them in the Position struct. Both templating
the do_move function on whether dirty threats are needed and putting a
null-check branch for each use of dirty threats were slowdowns locally. Of
course, nothing prevents a future attempt at cleaning this up.

passed STC
LLR: 2.96 (-2.94,2.94) <0.00,2.00>
Total: 68448 W: 17770 L: 17418 D: 33260
Ptnml(0-2): 198, 7425, 18630, 7769, 202
https://tests.stockfishchess.org/tests/view/6914c01a7ca87818523318ba

closes https://github.com/official-stockfish/Stockfish/pull/6414

No functional change

src/nnue/nnue_accumulator.cpp

@@ -21,6 +21,7 @@
 #include <algorithm>
 #include <cassert>
 #include <cstdint>
+#include <new>
 #include <type_traits>
 
 #include "../bitboard.h"
@@ -122,11 +123,13 @@ void AccumulatorStack::reset() noexcept {
     size = 1;
 }
 
-void AccumulatorStack::push(const DirtyBoardData& dirtyBoardData) noexcept {
+std::pair<DirtyPiece&, DirtyThreats&> AccumulatorStack::push() noexcept {
     assert(size < MaxSize);
-    psq_accumulators[size].reset(dirtyBoardData.dp);
-    threat_accumulators[size].reset(dirtyBoardData.dts);
+    auto& dp  = psq_accumulators[size].reset();
+    auto& dts = threat_accumulators[size].reset();
+    new (&dts) DirtyThreats;
     size++;
+    return {dp, dts};
 }
 
 void AccumulatorStack::pop() noexcept {

src/nnue/nnue_accumulator.h

@@ -25,6 +25,7 @@
 #include <cstddef>
 #include <cstdint>
 #include <cstring>
+#include <utility>
 
 #include "../types.h"
 #include "nnue_architecture.h"
@@ -141,6 +142,12 @@ struct AccumulatorState {
         accumulatorBig.computed.fill(false);
         accumulatorSmall.computed.fill(false);
     }
+
+    typename FeatureSet::DiffType& reset() noexcept {
+        accumulatorBig.computed.fill(false);
+        accumulatorSmall.computed.fill(false);
+        return diff;
+    }
 };
 
 class AccumulatorStack {
@@ -150,9 +157,10 @@ class AccumulatorStack {
     template<typename T>
     [[nodiscard]] const AccumulatorState<T>& latest() const noexcept;
 
-    void reset() noexcept;
-    void push(const DirtyBoardData& dirtyBoardData) noexcept;
-    void pop() noexcept;
+    void                                  reset() noexcept;
+    void                                  push(const DirtyBoardData& dirtyBoardData) noexcept;
+    std::pair<DirtyPiece&, DirtyThreats&> push() noexcept;
+    void                                  pop() noexcept;
 
     template<IndexType Dimensions>
     void evaluate(const Position&                       pos,