Improve error message on missing Immutable

[joshlf]

Per #1288 (comment):

The compiler errors when a type is missing Immutable are somewhat misleading, although I'm not sure there's much zerocopy can do about this. An example where the compiler recommends adding a reference rather than implementing Immutable (some were even more confusing than this):

error[E0277]: the trait bound `virtio::wl::CtrlVfdNewDmabuf: zerocopy::Immutable` is not satisfied                                          
   --> devices/src/virtio/wl.rs:317:20                                                                                                      
    |                                                                                                                                       
317 |         .write_obj(ctrl_vfd_new_dmabuf)                                                                                               
    |          --------- ^^^^^^^^^^^^^^^^^^^ the trait `zerocopy::Immutable` is not implemented for `virtio::wl::CtrlVfdNewDmabuf`          
    |          |                                                                                                                            
    |          required by a bound introduced by this call                                                                                  
    |                                                                                                                                       
note: required by a bound in `virtio::descriptor_utils::Writer::write_obj`                                                                  
   --> devices/src/virtio/descriptor_utils.rs:536:25                                                                                        
    |                                                                                                                                       
536 |     pub fn write_obj<T: Immutable + IntoBytes>(&mut self, val: T) -> io::Result<()> {                                                 
    |                         ^^^^^^^^^ required by this bound in `Writer::write_obj`                                                       
help: consider borrowing here                                                                                                               
    |                                                                                                                                       
317 |         .write_obj(&ctrl_vfd_new_dmabuf)                                                                                              
    |                    +                                                                                                                  
317 |         .write_obj(&mut ctrl_vfd_new_dmabuf)                                                                                          
    |                    ++++                                                

Taking the compiler's suggestion results in a different error with a recommendation to remove the reference (back to the original code).

We might be able to improve this via #1168.