Add defaults to compare_exchange, small fix in printf. Disallow obviously wrong code that returns the pointer to a variable on the stack.

test/test_suite/safe/detect_invalid_deref_return.c3

@@ -0,0 +1,40 @@
+fn int[] test1()
+{
+	int[3] x;
+	return &x; // #error: local variable will be invalid
+}
+
+fn int* test2()
+{
+	int[3] x;
+	return &x[0]; // #error: local variable will be invalid
+}
+
+struct Abc
+{
+	int a;
+	int[3] b;
+}
+
+fn int* test3()
+{
+	Abc x;
+	return &x.a; // #error: local variable will be invalid
+}
+
+fn int* test4()
+{
+	Abc x;
+	return &x.b; // #error: local variable will be invalid
+}
+
+fn int* test5()
+{
+	Abc x;
+	return &x.b[0]; // #error: local variable will be invalid
+}
+
+fn int* test6()
+{
+	return &&1; // #error: temporary value will
+}