Merge cleaned-up ExpressionStringBuilder into `StringBuilderExtensi…

…ons` (devlooped#882)

* Convert `ExpressionStringBuilder` to extension methods
* Chain `StringBuilder` operations
* General-purpose `AppendCommaSeparated` method reduces code duplication
* Inline a few one-line methods
* Simplify enclosed comma-separated lists code pattern
* Refactor more existing code to use `AppendCommaSeparated`
* Use more regular method names
* Merge into `StringBuilderExtensions`
   ... after ensuring that the (previous) `ExpressionStringBuilder` code
   file only contains `AppendExpression` methods at the top level (every-
   thing else is turned into local functions).
* Inline event add/remove formatting code

src/Moq/ExpressionExtensions.cs

@@ -8,7 +8,7 @@
 using System.Linq;
 using System.Linq.Expressions;
 using System.Reflection;
-using System.Runtime.CompilerServices;
+using System.Text;
 
 using Moq.Properties;
 using Moq.Protected;
@@ -419,7 +419,7 @@ private static bool PartialMatcherAwareEval_ShouldEvaluate(Expression expression
 		/// </devdoc>
 		public static string ToStringFixed(this Expression expression)
 		{
-			return new ExpressionStringBuilder().Append(expression).ToString();
+			return new StringBuilder().AppendExpression(expression).ToString();
 		}
 	}
 }