Convert SqlCompilerConfiguration to readonly struct (#389)

* Convert `SqlCompilerConfiguration` to `readonly struct`

* in

Author: SergeiPavlov

Orm/Xtensive.Orm/Sql/Compiler/SqlCompilerConfiguration.cs

@@ -17,30 +17,30 @@ namespace Xtensive.Sql.Compiler
   /// <summary>
   /// A various options for <see cref="SqlCompiler"/>.
   /// </summary>
-  public sealed class SqlCompilerConfiguration
+  public readonly struct SqlCompilerConfiguration()
   {
     /// <summary>
     /// Gets or sets the parameter prefix.
     /// </summary>
-    public string ParameterNamePrefix { get; set; }
+    public string ParameterNamePrefix { get; init; }
 
     /// <summary>
     /// Always use database-qualified objects in generated SQL.
     /// This option could be enabled if and only if
     /// server supports <see cref="QueryFeatures.MultidatabaseQueries"/>.
     /// </summary>
-    public bool DatabaseQualifiedObjects { get; set; }
+    public bool DatabaseQualifiedObjects { get; init; }
 
     /// <summary>
     /// Insert Placeholder nodes instead of real schema names
     /// Allows to share compiled query over multiple schemas.
     /// </summary>
-    public bool ParametrizeSchemaNames { get; set; }
+    public bool ParametrizeSchemaNames { get; init; }
 
     /// <summary>
     /// Gets or sets comment location.
     /// </summary>
-    public SqlCommentLocation CommentLocation { get; set; } = SqlCommentLocation.Nowhere;
+    public SqlCommentLocation CommentLocation { get; init; } = SqlCommentLocation.Nowhere;
 
     /// <summary>
     /// Clones this instance.

Orm/Xtensive.Orm/Sql/SqlDriver.cs

@@ -72,10 +72,8 @@ public SqlCompilationResult Compile(ISqlCompileUnit statement)
     /// <param name="configuration">The options of compilation.</param>
     /// <param name="typeIdRegistry">TypeId registry.</param>
     /// <returns>Result of compilation.</returns>
-    public SqlCompilationResult Compile(ISqlCompileUnit statement, SqlCompilerConfiguration configuration, TypeIdRegistry typeIdRegistry = null)
+    public SqlCompilationResult Compile(ISqlCompileUnit statement, in SqlCompilerConfiguration configuration, TypeIdRegistry typeIdRegistry = null)
     {
-      ArgumentNullException.ThrowIfNull(statement);
-      ArgumentNullException.ThrowIfNull(configuration);
       ValidateCompilerConfiguration(configuration);
       return CreateCompiler().Compile(statement, configuration);
     }
@@ -471,7 +469,7 @@ private async Task<Extractor> BuildExtractorAsync(SqlConnection connection, Canc
       return extractor;
     }
 
-    private void ValidateCompilerConfiguration(SqlCompilerConfiguration configuration)
+    private void ValidateCompilerConfiguration(in SqlCompilerConfiguration configuration)
     {
       var supported = ServerInfo.Query.Features.Supports(QueryFeatures.MultidatabaseQueries);
       var requested = configuration.DatabaseQualifiedObjects;