Macro shoebill::inter_trailing

macro_rules! inter_trailing {
    ( [], $sep:expr, $ctx:expr ) => { ... };
    ( [$d:expr], $sep:expr, $ctx:expr ) => { ... };
    ( [$d1:expr, $d2:expr], $sep:expr, $ctx:expr ) => { ... };
    ( [$d1:expr, $d2:expr, $($tl:expr),*], $sep:expr, $ctx:expr) => { ... };
}

Intercalate, adding a trailing sep chracter as well.