fix items for cstring for the JS target; makes tests green again

lib/system.nim

@@ -2195,10 +2195,17 @@ iterator items*[T](a: set[T]): T {.inline.} =
 
 iterator items*(a: cstring): char {.inline.} =
   ## iterates over each item of `a`.
-  var i = 0
-  while a[i] != '\0':
-    yield a[i]
-    inc(i)
+  when defined(js):
+    var i = 0
+    var L = len(a)
+    while i < L:
+      yield a[i]
+      inc(i)
+  else:
+    var i = 0
+    while a[i] != '\0':
+      yield a[i]
+      inc(i)
 
 iterator mitems*(a: var cstring): var char {.inline.} =
   ## iterates over each item of `a` so that you can modify the yielded value.

tests/js/tseqops.nim

@@ -1,8 +1,8 @@
 discard """
   output: '''(x: 0, y: 0)
 (x: 5, y: 0)
-@[(x: 2, y: 4), (x: 4, y: 5), (x: 4, y: 5)]
-@[(a: 3, b: 3), (a: 1, b: 1), (a: 2, b: 2)]
+@[(x: "2", y: 4), (x: "4", y: 5), (x: "4", y: 5)]
+@[(a: "3", b: 3), (a: "1", b: 1), (a: "2", b: 2)]
 '''
 """