Macro objects

macro_rules! objects {
    ( [$( $x_old:ident ),*], [$( $x:ident ),*] ) => { ... };
}

Generate per-version support code.

Here we have to deal with the fact that the pre-v79 objects had a protobuf-generated format, which gives them additional levels of indirection that the post-v79 objects don’t have and thus requires slightly different code to be generated.