Extending D's support for object-oriented design with private(this)

[Nick Treleaven]

It should say whether __traits(getMember), allMembers and 
.tupleof can access private(this) members.

private(this) maybe should be the default for synchronized 
classes.

BTW regarding unit tests, I would like to have `public unittest` 
disallow access to anything non public. Perhaps if accepted, 
there could later be private(this) to opt-in to accessing those 
fields. That is important when the class is a template and you 
don't want the test instantiated every time the template is 
instantiated.