Improve performance of map_row()
Improve performance of map_row()

Before:

    test bench_solve100  ... bench:      64,727 ns/iter (+/- 691)
    test bench_solve1000 ... bench:   7,160,735 ns/iter (+/- 315,280)

After:

    test bench_solve100  ... bench:      59,998 ns/iter (+/- 4,502)
    test bench_solve1000 ... bench:   6,761,411 ns/iter (+/- 379,337)

mneumann/munkres-rs