Fix missing assignment operator for Bitset

[songqing]

What do these changes do?

In PR #154 and #149, copy and move ctor for Bitset are added, however, the assignment operator is not added, and there will be compile error, see follow.

[codecov-commenter]

Codecov Report

Merging #155 (78f81a0) into master (45281c4) will not change coverage. Report is 4 commits behind head on master. The diff coverage is n/a.

Additional details and impacted files

[](https://app.codecov.io/gh/alibaba/libgrape-lite/pull/155?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=alibaba) ```diff @@ Coverage Diff @@ ## master #155 +/- ## ======================================= Coverage 63.96% 63.96% ======================================= Files 67 67 Lines 6310 6310 ======================================= Hits 4036 4036 Misses 2274 2274 ``` | [Flag](https://app.codecov.io/gh/alibaba/libgrape-lite/pull/155/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=alibaba) | Coverage Δ | | |---|---|---| | [app_tests](https://app.codecov.io/gh/alibaba/libgrape-lite/pull/155/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=alibaba) | `63.96% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=alibaba#carryforward-flags-in-the-pull-request-comment) to find out more. | [Files Changed](https://app.codecov.io/gh/alibaba/libgrape-lite/pull/155?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=alibaba) | Coverage Δ | | |---|---|---| | [grape/utils/bitset.h](https://app.codecov.io/gh/alibaba/libgrape-lite/pull/155?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=alibaba#diff-Z3JhcGUvdXRpbHMvYml0c2V0Lmg=) | `75.00% <ø> (ø)` | | | [grape/utils/concurrent\_queue.h](https://app.codecov.io/gh/alibaba/libgrape-lite/pull/155?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=alibaba#diff-Z3JhcGUvdXRpbHMvY29uY3VycmVudF9xdWV1ZS5o) | `90.62% <ø> (ø)` | |