Use `test::black_box` instead of xorring a var in the benchmarks.

For the 1 to 1 million benchmark, all 3 implementations (better_range,
std::iter::range, and while loop) run at the same speed now.

No change in the 1 to 10 million step 10 benchmark.

running 3 tests
test benches::while_loop_1_to_1_million                 ... bench:    298322 ns/iter (+/- 26995)
test benches::native_range_1_to_1_million               ... bench:    296930 ns/iter (+/- 9267)
test benches::better_range_from_1_to_1_million          ... bench:    297082 ns/iter (+/- 25564)

running 3 tests
test benches::while_loop_1_to_10_million_step_10        ... bench:    299082 ns/iter (+/- 23376)
test benches::native_range_step_1_to_10_million_step_10 ... bench:    891190 ns/iter (+/- 82721)
test benches::better_range_from_1_to_10_million_step_10 ... bench:   1045806 ns/iter (+/- 45712)