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.

src/Spectre.Console/IHasTreeNodes.cs

@@ -1,4 +1,4 @@
-using System.Collections.Generic;
+using System.Collections.Generic;
 
 namespace Spectre.Console
 {
@@ -8,8 +8,8 @@ namespace Spectre.Console
     public interface IHasTreeNodes
     {
         /// <summary>
-        /// Gets the children of this node.
+        /// Gets the tree's child nodes.
         /// </summary>
-        public List<TreeNode> Children { get; }
+        List<TreeNode> Nodes { get; }
     }
-}
+}