Issue an error if the user has nested foreach statements.

Partially addresses issue #280.  (We should support them properly,
but at least now we don't silently generate incorrect code.)

tests_errors/nested-foreach.ispc

@@ -0,0 +1,11 @@
+// Nested "foreach" statements are currently illegal
+
+void func();
+
+void foo(uniform int a, uniform int b) {
+    foreach (i = 0 ... a) {
+        foreach (j = 0 ... b) {
+            func();
+        }
+    }
+}