add function stack_push

just like the tittle. Thanks for the review. ;D

stack.c

@@ -17,9 +17,9 @@ void stack_destroy(stack *s) {
 
 void stack_push(stack *s, int value)
 {
-    if (s->top == s->capacity-1)
+    if (s->top == DEFAULT_CAPACITY-1)
     {
-        assert(s->data = realloc(s->capacity * 2 * sizeof(int)));
+        assert(s->data = realloc(DEFAULT_CAPACITY * 2 * sizeof(int)));
     }
 
     s->top++;