Added union method to HLL

[LucaCappelletti94]

• While several other methods relative to the joint estimation were available, union estimation was not.
• Added union method and relative test suite addition.

[LucaCappelletti94]

@ctb this one should be more straightforward than PR #3292 as it does not introduce any new dependency.

[ctb]

@luizirber is this something you can review?

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 86.74%. Comparing base (bc22970) to head (ed67c55). Report is 69 commits behind head on latest.

Files with missing lines	Patch %	Lines
src/core/src/sketch/hyperloglog/mod.rs	66.66%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## latest #3293 +/- ## ========================================== - Coverage 86.75% 86.74% -0.02% ========================================== Files 137 137 Lines 15920 15923 +3 Branches 2728 2728 ========================================== + Hits 13812 13813 +1 - Misses 1799 1801 +2 Partials 309 309 ``` | [Flag](https://app.codecov.io/gh/sourmash-bio/sourmash/pull/3293/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sourmash-bio) | Coverage Δ | | |---|---|---| | [hypothesis-py](https://app.codecov.io/gh/sourmash-bio/sourmash/pull/3293/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sourmash-bio) | `25.40% <ø> (ø)` | | | [python](https://app.codecov.io/gh/sourmash-bio/sourmash/pull/3293/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sourmash-bio) | `92.37% <ø> (ø)` | | | [rust](https://app.codecov.io/gh/sourmash-bio/sourmash/pull/3293/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sourmash-bio) | `62.64% <66.66%> (-0.03%)` | :arrow_down: | 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=sourmash-bio#carryforward-flags-in-the-pull-request-comment) to find out more.

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[luizirber]

Thanks @LucaCappelletti94 !