Add stack_print(stack) function

Addresses #12 (closed)

stack.c

@@ -18,6 +18,8 @@ void stack_peek(stack s, int *value){
 }
 
 void stack_print(const stack s) {
+	//TODO: replace if statement with following as soon as relevant function is implemented
+	//if (!stack_is_empty()) {
 	if (s.top >= 0) {
 		printf("          TOP\n--------------------\n");
 		for (int* spot = s.data + s.top; spot >= s.data; --spot) {