fixes mitems iterator on vm. (issue with tuple value)

this discard any implicit conversion on iterator return value.
the previous behaviour was to return a reference to a converted copy and
thus not modifying the original value.

compiler/semexprs.nim

@@ -1489,6 +1489,9 @@ proc semYieldVarResult(c: PContext, n: PNode, restype: PType) =
   var t = skipTypes(restype, {tyGenericInst, tyAlias})
   case t.kind
   of tyVar:
+    if n.sons[0].kind in {nkHiddenStdConv, nkHiddenSubConv}:
+      n.sons[0] = n.sons[0].sons[1]
+
     n.sons[0] = takeImplicitAddr(c, n.sons[0])
   of tyTuple:
     for i in 0.. <t.sonsLen: