dom-add-class/remove-class: handle list-of-elements targets

'add .foo to my children' compiles to (dom-add-class (host-get me 'children') 'foo') where
children is a list. Fanned out via for-each inside dom-add-class/dom-remove-class rather
than calling .classList.add on the list itself. Net: add 10→13.

shared/static/wasm/sx/dom.sx

@@ -261,12 +261,28 @@
       dom-add-class
       (fn
         (el cls)
-        (when el (host-call (host-get el "classList") "add" cls))))
+        (cond
+          ((nil? el) nil)
+          ((list? el)
+            (for-each
+              (fn
+                (x)
+                (when x (host-call (host-get x "classList") "add" cls)))
+              el))
+          (true (host-call (host-get el "classList") "add" cls)))))
     (define
       dom-remove-class
       (fn
         (el cls)
-        (when el (host-call (host-get el "classList") "remove" cls))))
+        (cond
+          ((nil? el) nil)
+          ((list? el)
+            (for-each
+              (fn
+                (x)
+                (when x (host-call (host-get x "classList") "remove" cls)))
+              el))
+          (true (host-call (host-get el "classList") "remove" cls)))))
     (define
       dom-has-class?
       (fn