'var T' for iterators

tests/accept/run/tmoditer.nim

@@ -0,0 +1,29 @@
+discard """
+  output: "XXXXX01234"
+"""
+
+iterator modPairs(a: var array[0..4,string]): tuple[key: int, val: var string] =
+  for i in 0..a.high:
+    yield (i, a[i])
+
+iterator modItems*[T](a: var array[0..4,T]): var T =
+  for i in 0..a.high:
+    yield a[i]
+
+var
+  arr = ["a", "b", "c", "d", "e"]
+
+for a in modItems(arr):
+  a = "X"
+
+for a in items(arr):
+  stdout.write(a)
+
+for i, a in modPairs(arr):
+  a = $i
+
+for a in items(arr):
+  stdout.write(a)
+
+echo ""
+