[engine.i] gnc_get_match_commodity_splits from scheme to c++

The scheme version is very inefficient: (1) it uses QofQuery to
retrieve all splits, and QofQuery is slow. (2) The whole splitlist
typically is thousands splits long needs to be converted from C to
guile via consing and reverse. (3) The scheme list is immediately
filtered to select "interesting" splits only.

This function performs all of the above using Account's efficient
methods, selecting only interesting splits into a GList. This list is
then converted to scheme via cons and reverse.

bindings/engine.i

@@ -41,6 +41,7 @@
 #include "gnc-kvp-guile.h"
 #include "glib-guile.h"
 
+#include "Account.hpp"
 #include "gncAddress.h"
 #include "gncBillTerm.h"
 #include "gncCustomer.h"
@@ -56,6 +57,13 @@
 %}
 #if defined(SWIGGUILE) //Always C++
 %{
+
+using SplitsVec = std::vector<Split*>;
+using AccountVec = std::vector<Account*>;
+
+SplitsVec gnc_get_match_commodity_splits (AccountVec accounts, bool use_end_date,
+                                          time64 end_date, gnc_commodity *comm, bool sort);
+
 extern "C"
 {
 SCM scm_init_sw_engine_module (void);
@@ -117,6 +125,28 @@ static const GncGUID * gncPriceGetGUID(GNCPrice *x)
 { return qof_instance_get_guid(QOF_INSTANCE(x)); }
 static const GncGUID * gncBudgetGetGUID(GncBudget *x)
 { return qof_instance_get_guid(QOF_INSTANCE(x)); }
+
+SplitsVec gnc_get_match_commodity_splits (AccountVec accounts, bool use_end_date,
+                                          time64 end_date, gnc_commodity *comm, bool sort)
+{
+    auto match = [use_end_date, end_date, comm](const Split* s) -> bool
+    {
+        if (xaccSplitGetReconcile (s) == VREC) return false;
+        auto trans{xaccSplitGetParent (s)};
+        if (use_end_date && xaccTransGetDate(trans) > end_date) return false;
+        auto txn_comm{xaccTransGetCurrency (trans)};
+        auto acc_comm{xaccAccountGetCommodity (xaccSplitGetAccount (s))};
+        return (txn_comm != acc_comm) && (!comm || comm == txn_comm || comm == acc_comm);
+    };
+    std::vector<Split*> rv;
+    auto maybe_accumulate_split = [&rv, match](auto s){ if (match(s)) rv.push_back (s); };
+    for (const auto acc : accounts)
+        gnc_account_foreach_split (acc, maybe_accumulate_split, true);
+    if (sort)
+        std::sort (rv.begin(), rv.end(), [](auto a, auto b){ return xaccSplitOrder (a, b) < 0; });
+    return rv;
+}
+
 %}
 
 /* NB: The object ownership annotations should already cover all the