Macro unnest_tuple

macro_rules! unnest_tuple {
    ($a: expr => $v: expr) => { ... };
    ($a: expr, $b: expr => $v: expr) => { ... };
    ($a: expr, $b: expr, $c: expr => $v: expr) => { ... };
    ($a: expr, $b: expr, $c: expr, $d: expr => $v: expr) => { ... };
    ($a: expr, $b: expr, $c: expr, $d: expr, $e: expr => $v: expr) => { ... };
    ($a: expr, $b: expr, $c: expr, $d: expr, $e: expr, $f: expr => $v: expr) => { ... };
}

(a, (b, c)) => (a, b, c)