Macro define_replica 

macro_rules! define_replica {
    ($(#[$m:meta])* $name:ident, $crypto:ty, $sb:expr) => { ... };
    ($(#[$m:meta])* $name:ident, external $sb:expr) => { ... };
    (@impl $(#[$m:meta])* $name:ident, $crypto:ty, $sb:expr, $ext:expr) => { ... };
}

Define a replica type $name<N, G> for signature scheme $crypto with $sb share beats.