Access struct fields from Roto

[tertsdiepraam]

Roto should be able to read and write fields from Rust types. These fields need to be registered with their offsets and types. This should be done with a declarative macro like so:

rt.register_clone_type(Foo, fields!{ Foo: bar, baz });

The fields macro would expand to something like:

unsafe { Fields::new(TypeId::of<Foo>(), &[
   Field::new("bar", std::mem::offset_of!(Foo.bar), create_type_description!(Foo, bar)),
   Field::new("baz", std::mem::offset_of!(Foo.baz), create_type_description!(Foo, baz)),
]) };

where create_type_description would be implemented like:

fn create_type_description_from_option<T>(x: Option<T>) -> ...{
    // do the reflect thing
}

macro_rules! create_type_description {
    ($ty: ident, $field: ident) => {{
        let opt: Option<$ty> = None;
        let opt = opt.map(|x| x.$field;
        create_type_description_from_option(opt)
    }}
}

Similarly, there should be a way to get enum variants into Roto.