EXP: avoid cloning in gather, use iterators directly

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 58.82353% with 14 lines in your changes missing coverage. Please review.

Project coverage is 86.36%. Comparing base (de1a597) to head (6f183dc). Report is 2 commits behind head on latest.

Files with missing lines	Patch %	Lines
src/core/src/sketch/minhash.rs	51.85%	13 Missing :warning:
src/core/src/index/mod.rs	75.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## latest #3394 +/- ## ========================================== - Coverage 86.47% 86.36% -0.11% ========================================== Files 137 137 Lines 16092 16106 +14 Branches 2219 2219 ========================================== - Hits 13915 13910 -5 - Misses 1870 1889 +19 Partials 307 307 ``` | [Flag](https://app.codecov.io/gh/sourmash-bio/sourmash/pull/3394/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/3394/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sourmash-bio) | `25.43% <ø> (ø)` | | | [python](https://app.codecov.io/gh/sourmash-bio/sourmash/pull/3394/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sourmash-bio) | `92.32% <ø> (-0.08%)` | :arrow_down: | | [rust](https://app.codecov.io/gh/sourmash-bio/sourmash/pull/3394/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sourmash-bio) | `62.15% <58.82%> (-0.12%)` | :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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