Macro spec_const_list

macro_rules! spec_const_list {
    (@ $(,)?) => { ... };
    (@ $head_id:expr => $head_constant:expr $(,$tail_id:expr => $tail_constant:expr)* $(,)?) => { ... };
    ($($id:expr => $constant:expr),* $(,)?) => { ... };
    ($($constant:expr),* $(,)?) => { ... };
}

Macro for specifying list of specialization constants for EntryPoint.