Macro once_chain

macro_rules! once_chain {
    (
        $first:expr
        $(
            , $second:expr
        )* $(,)?
    ) => { ... };
}

Macro to not repeat yourself writing once(...).chain(once(...))

Allows usage of calling one at a time:

const_str!(1 + 1)

or multiple at a time to even save repeated once_chain! invokations:

const_str! {
    1 + 1,
    2 + 2,
}