[examples] Consider using pointer or non pointer for the Tree type when using p/demo/avl

[jeronimoalbi]

Description

The p/demo/avl package has a constructor function NewTree() that returns a pointer *Tree. In may cases this function is not used because the Tree type can also be defined as a non pointer and called without the need of using the constructor function, for example:

package main

import "gno.land/p/demo/avl"

func main() {
  // Non pointer type
  var treeA avl.Tree
  println(treeA.Size())

  // Pointer type
  var treeB *avl.Tree = avl.NewTree()
  println(treeB.Size())
}

// Output:
// 0
// 0

This is currently possible because the underlying Node type that Tree uses is implemented in a way that handles all relevant nil cases with defaults.

Using a non pointer for the Tree type is convenient for example to have tree variables/properties initialized by default without the need to call NewTree().

It might be worth considering if the avl package, which is widely used, should favor and maybe settle for a pointer or non pointer approach by changing the package implementation to that purpose.

The downside of settling in one of the approaches is that it could be a breaking change.

[jeronimoalbi]

cc @leohhhn @moul @x1unix