higher order function for mapping via breadth-fisrt search
breadth-first map iterates by top-to-bottom search.
enum t = tuple(1, tuple( tuple( 4, tuple( 7)), 2), tuple( 3, tuple( 5, 6))); static assert(breadthFirstFlatMap!(x => x)(t) == tuple(1, 2, 3, 4, 5, 6, 7));
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higher order function for mapping via breadth-fisrt search