Added default activation dictionary

Results in a speed perf of 4 - 5%, but that's not where we're going
to see really good improvements.

SubSonic.Core/Linq/Structure/ExecutionBuilder.cs

@@ -20,6 +20,8 @@ namespace SubSonic.Linq.Structure
     /// </summary>
     public class ExecutionBuilder : DbExpressionVisitor
     {
+        private static IDictionary<Type, Expression> defaultValueActiviationDictionary = new Dictionary<Type, Expression>();
+
         private readonly List<Expression> initializers = new List<Expression>();
         private readonly QueryPolicy policy;
         private readonly Expression provider;
@@ -298,7 +300,12 @@ protected override Expression VisitColumn(ColumnExpression column)
                 }
                 else
                 {
-                    defvalue = Expression.Constant(Activator.CreateInstance(column.Type), column.Type);
+                    // Using a dictionary to cache default types increases performance 4 - 5%
+                    if (!defaultValueActiviationDictionary.TryGetValue(column.Type, out defvalue))
+                    {
+                        defvalue = Expression.Constant(Activator.CreateInstance(column.Type), column.Type);
+                        defaultValueActiviationDictionary.Add(column.Type, defvalue);
+                    }
                 }
 
                 // this sucks, but since we don't track true SQL types through the query, and ADO throws exception if you