C'tor for structs

[kiselgra]

Would be cool to extend cm-c:struct in cm-cxx to also support constructors.

We can, of course, just use class for the moment :)

[kiselgra]

When extending this, shoud we add base-classes to structs? This would make code using structs incompatible between cm-c and cm-c++.

I think we have to define a c++ version of struct anyhow (to supply the c'tor macrolet), so we could also check for something like this:

(struct foo :basees ((public bar) (private baz))
   body...)