Macro changeset

macro_rules! changeset {
    (checkpoints: $($tail:tt)*) => { ... };
    (
        index: $ind:ty,
        checkpoints: [ $(( $height:expr, $cp_to:expr )),* ]
        $(,txids: [ $(( $txid:expr, $tx_to:expr )),* ])?
    ) => { ... };
}