The core functionalities are now in two general functions
`extremeInSubtreeOnDirection()` and `nextOnDirection()` so all the other
traversing functions (`getMin()`, `getMax()`, and `InorderIterator`) are
all just trivial calls to these core functions.
The added two functions `Node.next()` and `Node.prev()` are also just
trivial calls to these.
* std.Treap traversal direction: use u1 instead of usize.
* Treap: fix getMin() and getMax(), and add tests for them.