Added `ArcStr::repeat` and `ArcStr::try_repeat`

[Kixiron]

Adds the ArcStr::repeat and ArcStr::try_repeat functions which are morally equivalent to String::repeat, allows users to avoid an intermediate String allocation in some circumstances

[codecov-commenter]

Codecov Report

Base: 92.24% // Head: 92.48% // Increases project coverage by +0.24% :tada:

Coverage data is based on head (9b37f19) compared to base (72d9d0b). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #50 +/- ## ========================================== + Coverage 92.24% 92.48% +0.24% ========================================== Files 4 4 Lines 374 386 +12 ========================================== + Hits 345 357 +12 Misses 29 29 ``` | [Impacted Files](https://codecov.io/gh/thomcc/arcstr/pull/50?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Thom+Chiovoloni) | Coverage Δ | | |---|---|---| | [src/arc\_str.rs](https://codecov.io/gh/thomcc/arcstr/pull/50/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Thom+Chiovoloni#diff-c3JjL2FyY19zdHIucnM=) | `92.10% <100.00%> (+0.43%)` | :arrow_up: | | [src/substr.rs](https://codecov.io/gh/thomcc/arcstr/pull/50/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Thom+Chiovoloni#diff-c3JjL3N1YnN0ci5ycw==) | `94.32% <0.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Thom+Chiovoloni). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Thom+Chiovoloni)

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[thomcc]

You need to put the test in the integration test folder. Under #[cfg(loom)] all the atomics and such become weird loom shit, which is broken to use outside loom::model. You could also put your tests behind #[cfg(not(loom))], but just put them with the others.

[thomcc]

in 1.1.5