[report-utilities] can strify records

srfi-9 records can contain complex objects eg lists/vectors also
gnc:monetary or gnc:html-table objects. previously gnc:strify would
use the default printer; this commit modifies so that they are
prettified.

example output; a :col-datum record from balsheet-pnl. the record's
split-balance contains a $0 monetary object.

Rec::col-datum{last-split=#f, split-balance=[$0.00]}

this last pretty-printer must be the last one before object->string,
because we want previous printers which may be records too
eg. monetary->str etc to use their own printer.

gnucash/report/report-system/report-utilities.scm

@@ -1284,6 +1284,13 @@ flawed. see report-utilities.scm. please update reports.")
       (try owner->str)
       (try invoice->str)
       (try lot->str)
+      (and (record? d)
+           (let ((rtd (record-type-descriptor d)))
+             (define (fld->str fld)
+               (format #f "~a=~a" fld (gnc:strify ((record-accessor rtd fld) d))))
+             (format #f "Rec:~a{~a}"
+                     (record-type-name rtd)
+                     (string-join (map fld->str (record-type-fields rtd)) ", "))))
       (object->string d)))
 
 (define (pair->num pair)