Use `allunique`

[devmotion]

This PR contains a minor optimization: Instead of computing unique(x) and comparing its length with the length of x, it just checks allunique(x).

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (932eaac) 93.75% compared to head (37ff520) 93.76%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #302 +/- ## ======================================= Coverage 93.75% 93.76% ======================================= Files 28 28 Lines 1729 1731 +2 ======================================= + Hits 1621 1623 +2 Misses 108 108 ``` | [Impacted Files](https://app.codecov.io/gh/JuliaStats/HypothesisTests.jl/pull/302?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaStats) | Coverage Δ | | |---|---|---| | [src/kolmogorov\_smirnov.jl](https://app.codecov.io/gh/JuliaStats/HypothesisTests.jl/pull/302?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaStats#diff-c3JjL2tvbG1vZ29yb3Zfc21pcm5vdi5qbA==) | `96.15% <100.00%> (ø)` | | ... and [2 files with indirect coverage changes](https://app.codecov.io/gh/JuliaStats/HypothesisTests.jl/pull/302/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaStats)

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[nalimilan]

Thanks!