[html-document] schemify gnc:html-document-tree-collapse

this function is technically a flattening function, converted to
classic scheme form. very efficient in time and space. it is used
extensively in reports which are still running well, therefore no
additional testing is required.

gnucash/report/report-system/html-document.scm

@@ -105,20 +105,11 @@
        (apply gnc:make-html-data-style-info rest)
        (apply gnc:make-html-markup-style-info rest))))
 
-(define (gnc:html-document-tree-collapse tree)
-  (let ((retval '()))
-    (let loop ((lst tree))
-      (for-each
-       (lambda (elt)
-         (cond
-          ((string? elt)
-           (set! retval (cons elt retval)))
-          ((not (list? elt))
-           (set! retval (cons (object->string elt) retval)))
-          (else
-           (loop elt))))
-       lst))
-    retval))
+(define (gnc:html-document-tree-collapse . tree)
+  (let lp ((e tree) (accum '()))
+    (cond ((list? e) (fold lp accum e))
+          ((string? e) (cons e accum))
+          (else (cons (object->string e) accum)))))
 
 ;; first optional argument is "headers?"
 ;; returns the html document as a string, I think.