Resolve "Add destroy function"

Closes #9 (closed)

stack.c

+#include <stdio.h>
 #include "stack.h"
+
+void stack_destroy(stack *s){
+    free(s->data);
+    s->data = NULL;
+    s->capacity = -1;
+    s->top = -1;
+}
\ No newline at end of file