[balance-forecast] refactor to avoid xaccAccountGetBalanceAsOfDate

previous will call gnc:account-get-comm-balance-at-date which calls
xaccAccountGetBalanceAsOfDate for every account at every date
point. The xaccAccountGetBalanceAsOfDate is an expensive function
because it scans the account's whole splitlist from the start every
time. use gnc:account-get-balances-at-dates instead which scans an
account only once to generate a balancelist.

this should be a much faster chart.

gnucash/report/standard-reports/balance-forecast.scm

@@ -27,6 +27,7 @@
 
 (use-modules (gnucash gnc-module))
 (use-modules (gnucash gettext))
+(use-modules (srfi srfi-1))
 
 (gnc:module-load "gnucash/report/report-system" 0)
 
@@ -142,6 +143,13 @@
           (intervals (gnc:make-date-interval-list
             from-date to-date (gnc:deltasym-to-delta interval)))
           (accum (gnc:make-commodity-collector))
+          (accounts-balancelist
+           (map
+            (lambda (acc)
+              (gnc:account-get-balances-at-dates acc (map cadr intervals)))
+            accounts))
+          (accounts-balancelist-transposed
+           (if (null? accounts) '() (apply zip accounts-balancelist)))
           (balances #f)
         )
 
@@ -162,23 +170,23 @@
 
     ; Calculate balances
     (set! balances
-      (map (lambda (date) (let* (
+      (map (lambda (date accounts-balance) (let* (
           (start-date (car date))
           (end-date (cadr date))
           (balance (gnc:make-commodity-collector))
           (exchange-fn (gnc:case-exchange-fn price currency end-date))
           (sx-value (gnc-sx-all-instantiate-cashflow-all start-date end-date))
          )
-        (for-each (lambda (account)
+        (for-each (lambda (account account-balance)
           (accum 'add (xaccAccountGetCommodity account)
             (hash-ref sx-value (gncAccountGetGUID account) 0))
-          (balance 'merge
-            (gnc:account-get-comm-balance-at-date account end-date #f) #f)
-        ) accounts)
+          (balance 'add (gnc:gnc-monetary-commodity account-balance)
+            (gnc:gnc-monetary-amount account-balance))
+        ) accounts accounts-balance)
         (balance 'merge accum #f)
         (gnc:gnc-monetary-amount
           (gnc:sum-collector-commodity balance currency exchange-fn))
-      )) intervals))
+      )) intervals accounts-balancelist-transposed))
 
     ; Minimum line
     (when show-minimum