Add functional variant of threefry

[jw3126]

In some contexts, mutable structs are problematic (for instance on GPU). This adds a low-level purely functional way to use threefry.
If there is interest in this, I will add functional variants of the other rngs as well.
I can also add a convenience function for functional counter increment if desired.

[jw3126]

@sunoru any thoughts on having a side effect-free low-level API?

[sunoru]

Sorry for replying late! Having such low-level APIs sounds useful. Can you also help add them to other RNGs?

[jw3126]

@sunoru I added the other rngs. Currently aesni and ars operate on __m128i which I am not sure we want to expose? Should I sandwich aesni and ars in conversion functions that let them operate on UInt128 instead?

[sunoru]

Sure I guess we can just also have a version of aesni and ars to operate on UInt128 values.

[jw3126]

I added such aesni and ars could you run CI?

[sunoru]

OK! BTW, I think it's OK to export these functions.

[codecov-commenter]

Codecov Report

Merging #15 (c9c5f3d) into master (73adf98) will increase coverage by 0.05%.
The diff coverage is 100.00%.

❗ Current head c9c5f3d differs from pull request most recent head f49cdfc. Consider uploading reports for the commit f49cdfc to get more accurate results

@@            Coverage Diff             @@
##           master      #15      +/-   ##
==========================================
+ Coverage   98.57%   98.63%   +0.05%     
==========================================
  Files           7        7              
  Lines         842      879      +37     
==========================================
+ Hits          830      867      +37     
  Misses         12       12              

Impacted Files	Coverage Δ
src/aesni.jl	99.09% <100.00%> (+0.13%)	⬆️
src/ars.jl	98.66% <100.00%> (+0.30%)	⬆️
src/philox.jl	98.63% <100.00%> (+0.08%)	⬆️
src/threefry.jl	99.54% <100.00%> (ø)

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[sunoru]

Looks good to me. Thanks very much!