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/Rendering/Tree/AsciiTreeAppearance.cs

@@ -1,25 +0,0 @@
-using System;
-
-namespace Spectre.Console.Rendering
-{
-    /// <summary>
-    /// An ASCII rendering of a tree.
-    /// </summary>
-    public sealed class AsciiTreeAppearance : TreeAppearance
-    {
-        /// <inheritdoc/>
-        public override int PartSize => 4;
-
-        /// <inheritdoc/>
-        public override string GetPart(TreePart part)
-        {
-            return part switch
-            {
-                TreePart.SiblingConnector => "│   ",
-                TreePart.ChildBranch => "├── ",
-                TreePart.BottomChildBranch => "└── ",
-                _ => throw new ArgumentOutOfRangeException(nameof(part), part, "Unknown tree part."),
-            };
-        }
-    }
-}

src/Spectre.Console/Rendering/Tree/AsciiTreeGuide.cs

@@ -0,0 +1,23 @@
+using System;
+
+namespace Spectre.Console.Rendering
+{
+    /// <summary>
+    /// An ASCII tree guide.
+    /// </summary>
+    public sealed class AsciiTreeGuide : TreeGuide
+    {
+        /// <inheritdoc/>
+        public override string GetPart(TreeGuidePart part)
+        {
+            return part switch
+            {
+                TreeGuidePart.Space => "    ",
+                TreeGuidePart.Continue => "|   ",
+                TreeGuidePart.Fork => "|-- ",
+                TreeGuidePart.End => "`-- ",
+                _ => throw new ArgumentOutOfRangeException(nameof(part), part, "Unknown tree part."),
+            };
+        }
+    }
+}

src/Spectre.Console/Rendering/Tree/BoldLineTreeGuide.cs

@@ -0,0 +1,26 @@
+using System;
+
+namespace Spectre.Console.Rendering
+{
+    /// <summary>
+    /// A tree guide made up of bold lines.
+    /// </summary>
+    public sealed class BoldLineTreeGuide : TreeGuide
+    {
+        /// <inheritdoc/>
+        public override TreeGuide? SafeTreeGuide => Ascii;
+
+        /// <inheritdoc/>
+        public override string GetPart(TreeGuidePart part)
+        {
+            return part switch
+            {
+                TreeGuidePart.Space => "    ",
+                TreeGuidePart.Continue => "┃   ",
+                TreeGuidePart.Fork => "┣━━ ",
+                TreeGuidePart.End => "┗━━ ",
+                _ => throw new ArgumentOutOfRangeException(nameof(part), part, "Unknown tree part."),
+            };
+        }
+    }
+}

src/Spectre.Console/Rendering/Tree/DoubleLineTreeGuide.cs

@@ -0,0 +1,26 @@
+using System;
+
+namespace Spectre.Console.Rendering
+{
+    /// <summary>
+    /// A tree guide made up of double lines.
+    /// </summary>
+    public sealed class DoubleLineTreeGuide : TreeGuide
+    {
+        /// <inheritdoc/>
+        public override TreeGuide? SafeTreeGuide => Ascii;
+
+        /// <inheritdoc/>
+        public override string GetPart(TreeGuidePart part)
+        {
+            return part switch
+            {
+                TreeGuidePart.Space => "    ",
+                TreeGuidePart.Continue => "║   ",
+                TreeGuidePart.Fork => "╠══ ",
+                TreeGuidePart.End => "╚══ ",
+                _ => throw new ArgumentOutOfRangeException(nameof(part), part, "Unknown tree part."),
+            };
+        }
+    }
+}

src/Spectre.Console/Rendering/Tree/LineTreeGuide.cs

@@ -0,0 +1,26 @@
+using System;
+
+namespace Spectre.Console.Rendering
+{
+    /// <summary>
+    /// A tree guide made up of lines.
+    /// </summary>
+    public sealed class LineTreeGuide : TreeGuide
+    {
+        /// <inheritdoc/>
+        public override TreeGuide? SafeTreeGuide => Ascii;
+
+        /// <inheritdoc/>
+        public override string GetPart(TreeGuidePart part)
+        {
+            return part switch
+            {
+                TreeGuidePart.Space => "    ",
+                TreeGuidePart.Continue => "│   ",
+                TreeGuidePart.Fork => "├── ",
+                TreeGuidePart.End => "└── ",
+                _ => throw new ArgumentOutOfRangeException(nameof(part), part, "Unknown tree part."),
+            };
+        }
+    }
+}

src/Spectre.Console/Rendering/TreeGuidePart.cs

@@ -3,21 +3,26 @@ namespace Spectre.Console.Rendering
     /// <summary>
     /// Defines the different rendering parts of a <see cref="Tree"/>.
     /// </summary>
-    public enum TreePart
+    public enum TreeGuidePart
     {
+        /// <summary>
+        /// Represents a space.
+        /// </summary>
+        Space,
+
         /// <summary>
         /// Connection between siblings.
         /// </summary>
-        SiblingConnector,
+        Continue,
 
         /// <summary>
         /// Branch from parent to child.
         /// </summary>
-        ChildBranch,
+        Fork,
 
         /// <summary>
         /// Branch from parent to child for the last child in a set.
         /// </summary>
-        BottomChildBranch,
+        End,
     }
 }