Function select6

pub fn select6<A, B, C, D, E, F>(
    a: A,
    b: B,
    c: C,
    d: D,
    e: E,
    f: F,
) -> Select6<A, B, C, D, E, F> 
where
    A: Future,
    B: Future,
    C: Future,
    D: Future,
    E: Future,
    F: Future,

Same as select, but with more futures.