Macro grid 

macro_rules! grid {
    ( $( $path:expr => [ $( $val:expr ),* $(,)? ] ),* $(,)? ) => { ... };
}

Build a ParamGrid with scikit-learn-style "step__param" => [values].

use millwright::grid;
let g = grid! {
    "rf__max_depth" => [4, 8, 16],
    "scale__with_mean" => [true, false],
};
assert_eq!(g.combinations().len(), 6);