using loop instead of linq

In both of these loops context is captured preventing caching of the lambda. this results in a pretty significant amount of allocations especially with progress bars that constantly are remeasuring

src/Spectre.Console/Internal/Cell.cs

@@ -8,7 +8,13 @@ namespace Spectre.Console
     {
         public static int GetCellLength(RenderContext context, string text)
         {
-            return text.Sum(rune => GetCellLength(context, rune));
+            var sum = 0;
+            foreach (var rune in text)
+            {
+                sum += GetCellLength(context, rune);
+            }
+
+            return sum;
         }
 
         public static int GetCellLength(RenderContext context, char rune)

src/Spectre.Console/Rendering/Segment.cs

@@ -135,7 +135,13 @@ namespace Spectre.Console.Rendering
                 throw new ArgumentNullException(nameof(segments));
             }
 
-            return segments.Sum(segment => segment.CellCount(context));
+            var sum = 0;
+            foreach (var segment in segments)
+            {
+                sum += segment.CellCount(context);
+            }
+
+            return sum;
         }
 
         /// <summary>