Allow marking a C++ class as sandboxed

[arichardson]

I would be nice if I could annotate a C++ class as sandboxed by using e.g. __soaap_sandboxed_class("sandboxname").
SOAAP should then treat every function in that class (including constructors and destructor) as running in a sandbox.

My current workaround is to add a dummy function that calls every method on the class (which can be quite a lot in some cases):

__soaap_sandbox_persistent("foo") soaap_dummy() {
  Foo foo(args);
  foo.bar();
  foo.baz(1234);
  // ....
}