relax isIterator constraints

Author: 9il

source/mir/ndslice/traits.d

@@ -232,8 +232,8 @@ template isIterator(T)
         --a;
         --b;
         auto t = a[sizediff_t(3)];
-        b = a + sizediff_t(3);
-        b = a - sizediff_t(3);
+        //b = a + sizediff_t(3);
+        //b = a - sizediff_t(3);
         a += sizediff_t(3);
         a -= sizediff_t(3);
         return *a;