Add foreach_unique iteration construct.

Idea via Ingo Wald / IVL compiler.

tests_errors/foreach-unique-4.ispc

@@ -0,0 +1,10 @@
+// Error: "return" statement is illegal inside a "foreach" loop
+
+void foo(int x) {
+    foreach_unique (u in x) {
+        if (u == 0)
+            return;
+    }
+}
+
+