Macro shared_vector::arc_vector

macro_rules! arc_vector {
    ($elem:expr; $n:expr) => { ... };
    ($($x:expr),*$(,)*) => { ... };
    ([$($x:expr),*$(,)*] in $allocator:expr) => { ... };
    ([$elem:expr;$n:expr] in $allocator:expr) => { ... };
}