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]

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

@@            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	Coverage Δ
ext/VectorizedRNGStaticArraysExt.jl	100.00% <100.00%> (ø)
src/api.jl	71.31% <100.00%> (+6.86%)	⬆️
src/VectorizedRNG.jl	82.14% <0.00%> (-3.05%)	⬇️

... and 1 file with indirect coverage changes

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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.