Try to get rid of com_impl

[Rantanen]

Getting rid of #[com_impl] attribute would...

• Simplify usage
• Allow attributing method signatures in a way that affects generated COM interfaces without the need to introduce these attributes in the trait impl.

Currently the VTable generation is tied to the com_impl. There's a chance that can be removed by generating the type conversion functions as generic functions:

extern "system" fn __IFoo_Automation_do_stuff<TStruct>(vtbl: *const *const VTable, params...)
where TStruct: ComInterface<IFoo>
{
    let instance_ptr = vtbl - <TStruct as ComInterface<IFoo>>::offset();
    let instance : &TStruct = &*vtbl;
    ...
}