Provide a procedural macro to cast structs onto the shmem

[elast0ny]

With the new procedural macros in 1.30, I think it would now be possible to have something like :

#[derive(SharedMemCast)]
struct MyStruct {
    //...
}

The procedural macro would recursively ensure that all the types in MyStruct are safe to be used in shared memory...

This would remove the need for SharedMemCast