Allow for defining trivial types in C++Qt blocks

Currently cxx-qt-lib's advantage is that we can define a trivial type as we alias the C++ type to a struct with a known size and assert it on either side.

What if we could do the following

#[cxx_qt::bridge]
mod ffi {
   unsafe extern "C++Qt" {
     #[cxx_qt_size(usize, usize, usize)] // maybe alignment too?
     type QString;
   }
}

Then could the bridge automatically generate the struct with the MaybeUninit and the assert on the C++ side?

There would then be questions over automatically implementing Clone/Drop/Default etc? Some of these should probably be attributes as they affect the movable asserts on the C++ side.