Built-in abs/min/max for integers; For-loop fixes

tests/compiler/for.ts

@@ -2,5 +2,19 @@ let i: i32;
 for (i = 0; i < 10; ++i) {
   ;
 }
-if (i != 10)
-  unreachable();
+assert(i == 10);
+
+for (let j: i32 = 0; j < 10; ++j) {
+  ;
+}
+
+for (; i > 0; --i);
+assert(i == 0);
+
+for (;; ++i)
+  if (i == 10)
+    break;
+
+for (;;)
+  if (--i == 0)
+    break;