make fri::Block.position public to build blocks

i'm building the CLI and it's annoying to not be able to build blocks...

this MR allows to write things like

Block {
    shard: serde::deser_shard(&id, *validate),
    proof: serde::deser_serializable(&proof, Compress::No, *validate),
    commit: Rc::new(commitment.clone()),
    position,
}