Fix tree rendering

Fixes some tree rendering problems where lines were not properly drawn
at some levels during some circumstances.

* Change the API back to only allow one root.
* Now uses a stack based approach to rendering instead of recursion.
* Removes the need for measuring the whole tree in advance.
  Leave this up to each child to render.

examples/Console/Prompt/Program.cs

@@ -1,4 +1,3 @@
-using System.Collections.Generic;
 using Spectre.Console;
 
 namespace Cursor