"Add get length function"

Closes #8 (closed)

stack.c

@@ -24,4 +24,9 @@ void stack_clone(stack s, stack *clone) {
     for (int i = 0; i <= s.top && i < s.capacity; i++) {
         clone->data[i] = s.data[i];
     }
-}
\ No newline at end of file
+}
+
+int get_length(stack *pile)
+{
+    return pile->capacity;
+}