fix #17159 items(cstring) works in VM (#17160)

* fix #17159 items(cstring) works in VM

* improve test coverage tests/stdlib/tcstring.nim; add helpers: whenRuntimeJs, whenVMorJs

* document items(cstring)

* address comments

lib/system/iterators.nim

@@ -56,31 +56,62 @@ iterator items*[T](a: set[T]): T {.inline.} =
 
 iterator items*(a: cstring): char {.inline.} =
   ## Iterates over each item of `a`.
-  when defined(js):
+  runnableExamples:
+    from std/sequtils import toSeq
+    assert toSeq("abc\0def".cstring) == @['a', 'b', 'c']
+    assert toSeq("abc".cstring) == @['a', 'b', 'c']
+  #[
+  assert toSeq(nil.cstring) == @[] # xxx fails with SIGSEGV
+  this fails with SIGSEGV; unclear whether we want to instead yield nothing
+  or pay a small price to check for `nil`, a benchmark is needed. Note that
+  other procs support `nil`.
+  ]#
+  template impl() =
     var i = 0
-    var L = len(a)
-    while i < L:
+    let n = len(a)
+    while i < n:
       yield a[i]
       inc(i)
+  when defined(js): impl()
   else:
-    var i = 0
-    while a[i] != '\0':
-      yield a[i]
-      inc(i)
+    when nimvm:
+      # xxx `cstring` should behave like c backend instead.
+      impl()
+    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.
-  when defined(js):
+  # xxx this should give CT error in js RT.
+  runnableExamples:
+    from std/sugar import collect
+    var a = "abc\0def"
+    var b = a.cstring
+    let s = collect:
+      for bi in mitems(b):
+        if bi == 'b': bi = 'B'
+        bi
+    assert s == @['a', 'B', 'c']
+    assert b == "aBc"
+    assert a == "aBc\0def"
+
+  template impl() =
     var i = 0
-    var L = len(a)
-    while i < L:
+    let n = len(a)
+    while i < n:
       yield a[i]
       inc(i)
+  when defined(js): impl()
   else:
-    var i = 0
-    while a[i] != '\0':
-      yield a[i]
-      inc(i)
+    when nimvm: impl()
+    else:
+      var i = 0
+      while a[i] != '\0':
+        yield a[i]
+        inc(i)
 
 iterator items*[T: enum and Ordinal](E: typedesc[T]): T =
   ## Iterates over the values of `E`.