Add `vhash-fold-right'.

* module/ice-9/vlist.scm (vhash-fold-right): New procedure.

* test-suite/tests/vlist.test ("vhash")["vhash-fold-right"]: New test.

* doc/ref/api-compound.texi (VHashes): Document `vhash-fold-right'.

module/ice-9/vlist.scm

@@ -33,7 +33,7 @@
             vhash? vhash-cons vhash-consq vhash-consv
             vhash-assoc vhash-assq vhash-assv
             vhash-delete vhash-delq vhash-delv
-            vhash-fold
+            vhash-fold vhash-fold-right
             vhash-fold* vhash-foldq* vhash-foldv*
             alist->vhash))
 
@@ -560,6 +560,16 @@ with @var{equal?}."
               seed
               vhash))
 
+(define (vhash-fold-right proc seed vhash)
+  "Fold over the key/pair elements of @var{vhash}, starting from the 0th
+element.  For each pair call @var{proc} as @code{(@var{proc} key value
+result)}."
+  (vlist-fold-right (lambda (key+value result)
+                      (proc (car key+value) (cdr key+value)
+                            result))
+                    seed
+                    vhash))
+
 (define* (alist->vhash alist #:optional (hash hash))
   "Return the vhash corresponding to @var{alist}, an association list."
   (fold-right (lambda (pair result)