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/TreeGuide.cs

@@ -0,0 +1,22 @@
+using Spectre.Console.Rendering;
+
+namespace Spectre.Console
+{
+    /// <summary>
+    /// Represents tree guide lines.
+    /// </summary>
+    public abstract partial class TreeGuide
+    {
+        /// <summary>
+        /// Gets the safe guide lines or <c>null</c> if none exist.
+        /// </summary>
+        public virtual TreeGuide? SafeTreeGuide { get; }
+
+        /// <summary>
+        /// Get the set of characters used to render the corresponding <see cref="TreeGuidePart"/>.
+        /// </summary>
+        /// <param name="part">The part of the tree to get rendering string for.</param>
+        /// <returns>Rendering string for the tree part.</returns>
+        public abstract string GetPart(TreeGuidePart part);
+    }
+}