corrected rand! for mutable static arrays and moved them to an extension

[schrimpf]

Fixes #27

A couple things of note:

• I removed methods for immutable SArrays, since I think it's just not possible to modify them in place
• I removed the dependency on StrideArraysCore. It was only used to create Ptr(x) and I don't think it's needed.

[chriselrod]

The point of the PtrArray is to potentially avoid allocations. Julia does not have proper escape analysis, so it conservatively heap allocates unnecessarily. PtrArray with GC.@preserve is a workaround.

MArray <: StaticArray and SArray <: StaticArray. MArray is mutable, but SArray is not. Other hypothetical StaticArrays are mutable.

I don't really use this with StaticArrays, though, and we can blame current limitations of Julia's compiler for the allocations, so I'm okay merging this.

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (c34cf44) 74.10% compared to head (bfe3c8f) 73.82%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #28 +/- ## ========================================== - Coverage 74.10% 73.82% -0.28% ========================================== Files 5 6 +1 Lines 695 680 -15 ========================================== - Hits 515 502 -13 + Misses 180 178 -2 ``` | [Files](https://app.codecov.io/gh/JuliaSIMD/VectorizedRNG.jl/pull/28?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaSIMD) | Coverage Δ | | |---|---|---| | [ext/VectorizedRNGStaticArraysExt.jl](https://app.codecov.io/gh/JuliaSIMD/VectorizedRNG.jl/pull/28?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaSIMD#diff-ZXh0L1ZlY3Rvcml6ZWRSTkdTdGF0aWNBcnJheXNFeHQuamw=) | `100.00% <100.00%> (ø)` | | | [src/api.jl](https://app.codecov.io/gh/JuliaSIMD/VectorizedRNG.jl/pull/28?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaSIMD#diff-c3JjL2FwaS5qbA==) | `71.31% <100.00%> (+6.86%)` | :arrow_up: | | [src/VectorizedRNG.jl](https://app.codecov.io/gh/JuliaSIMD/VectorizedRNG.jl/pull/28?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaSIMD#diff-c3JjL1ZlY3Rvcml6ZWRSTkcuamw=) | `82.14% <0.00%> (-3.05%)` | :arrow_down: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/JuliaSIMD/VectorizedRNG.jl/pull/28/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaSIMD)

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[schrimpf]

This last change is about the best I can do. I do not understand the failed Aqua tests on version 1.6, and cannot fix them. They do not seem to matter anyway.

[chriselrod]

This last change is about the best I can do. I do not understand the failed Aqua tests on version 1.6, and cannot fix them. They do not seem to matter anyway.

I wouldn't worry about them either.