General uncontroversial cleanup:

- Made private methods static where possible (runtime can elide checking the object for null).
- Declared attribute classes as sealed (allows reflection on attributes to complete faster).
- Moved some static cctor's into field initializers (static cctor's are slower than static field initializers).
- Made classes static if they contained only static methods (can't create instances of useless objects).
- Use inferable Exts.Lazy and not new Lazy<T>().
- Added required STAThread attribute to CrashDialog.
- Removed unused parameters in private methods.
- Added Serializable attribute to exceptions.
- Added parameter name in calls to ArgumentNullException.
- Use of as operator instead of is + cast.
- Changed (x as Foo).Bar anti-pattern into ((Foo)x).Bar. Results in sensible cast exceptions on error rather than null dereferences.
- Removed unused method in NullShader.

OpenRA.Game/FieldLoader.cs

@@ -402,10 +402,10 @@ namespace OpenRA
 		}
 
 		[AttributeUsage(AttributeTargets.Field)]
-		public class IgnoreAttribute : Attribute { }
+		public sealed class IgnoreAttribute : Attribute { }
 
 		[AttributeUsage(AttributeTargets.Field)]
-		public class LoadUsingAttribute : Attribute
+		public sealed class LoadUsingAttribute : Attribute
 		{
 			Func<MiniYaml, object> loaderFuncCache;
 			public readonly string Loader;
@@ -440,12 +440,12 @@ namespace OpenRA
 	}
 
 	[AttributeUsage(AttributeTargets.Field | AttributeTargets.Property)]
-	public class TranslateAttribute : Attribute { }
+	public sealed class TranslateAttribute : Attribute { }
 
-	public class FieldFromYamlKeyAttribute : Attribute { }
+	public sealed class FieldFromYamlKeyAttribute : Attribute { }
 
 	// mirrors DescriptionAttribute from System.ComponentModel but we dont want to have to use that everywhere.
-	public class DescAttribute : Attribute
+	public sealed class DescAttribute : Attribute
 	{
 		public readonly string[] Lines;
 		public DescAttribute(params string[] lines) { Lines = lines; }