Use array views in spin functions

[jishnub]

This should reduce some memory allocation

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: -3.66 :warning:

Comparison is base (3d6990d) 66.77% compared to head (7b108bb) 63.12%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2 +/- ## ========================================== - Coverage 66.77% 63.12% -3.66% ========================================== Files 4 4 Lines 301 301 ========================================== - Hits 201 190 -11 - Misses 100 111 +11 ``` | [Impacted Files](https://app.codecov.io/gh/eschnett/FastSphericalHarmonics.jl/pull/2?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Erik+Schnetter) | Coverage Δ | | |---|---|---| | [src/spin.jl](https://app.codecov.io/gh/eschnett/FastSphericalHarmonics.jl/pull/2?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Erik+Schnetter#diff-c3JjL3NwaW4uamw=) | `92.50% <100.00%> (-6.25%)` | :arrow_down: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/eschnett/FastSphericalHarmonics.jl/pull/2/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Erik+Schnetter)

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[eschnett]

Thank you! I was not aware of the convenient @views macro, and didn't spot the places where the previous code had unnecessary allocations.