Add unittest

Add unittest

[   19s] + ./bit_set
[   19s]
[   19s] running 22 tests
[   19s] test tests::test_bit_set_basic ... ok
[   19s] test tests::test_bit_set_difference ... ok
[   19s] test tests::test_bit_set_difference_with ... ok
[   19s] test tests::test_bit_set_eq ... ok
[   19s] test tests::test_bit_set_cmp ... ok
[   19s] test tests::test_bit_set_from_usizes ... ok
[   19s] test tests::test_bit_set_frombit_vec_init ... ok
[   19s] test tests::test_bit_set_intersection ... ok
[   19s] test tests::test_bit_set_is_disjoint ... ok
[   19s] test tests::test_bit_set_intersect_with ... ok
[   19s] test tests::test_bit_set_shrink_to_fit ... ok
[   19s] test tests::test_bit_set_show ... ok
[   19s] test tests::test_bit_set_subset ... ok
[   19s] test tests::test_bit_set_shrink_to_fit_new ... ok
[   19s] test tests::test_bit_set_symmetric_difference ... ok
[   19s] test tests::test_bit_set_iterator ... ok
[   19s] test tests::test_bit_set_symmetric_difference_with ... ok
[   19s] test tests::test_bit_set_union ... ok
[   19s] test tests::test_bit_set_union_with ... ok
[   19s] test tests::test_bit_vec_clone ... ok
[   19s] test tests::test_bit_vec_masking ... ok
[   19s] test tests::test_bit_vec_remove ... ok
[   19s]
[   19s] test result: ok. 22 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.04s