Resolve "Add clear function"

Closes #13 (closed)

stack.c

-#include <stdlib.h>
-#include <stdbool.h>
-#include <stdio.h>
-
 #include "stack.h"
 
 
@@ -87,4 +83,15 @@ int get_length(stack s) {
 bool stack_is_empty(stack s)
 {
     return s.top == -1;
+}
+
+/**
+ * @brief Reset the stack's top to -1
+ *        New values will overwrite the old ones 
+ * 
+ * @param s
+ */
+void stack_clear(stack *s) 
+{
+    s->top = -1;
 }
\ No newline at end of file