Use atomic_pointer* methods when available

[jpsamaroo]

Todo:

• Add modify! implementation

[codecov]

Codecov Report

Attention: Patch coverage is 47.36842% with 10 lines in your changes are missing coverage. Please review.

Project coverage is 81.66%. Comparing base (934c34d) to head (0953048).

Files	Patch %	Lines
src/core.jl	47.36%	10 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master      #14       +/-   ##
===========================================
- Coverage   98.00%   81.66%   -16.34%     
===========================================
  Files           4        4               
  Lines          50       60       +10     
===========================================
  Hits           49       49               
- Misses          1       11       +10     

Flag	Coverage Δ
unittests	81.66% <47.36%> (-16.34%)	⬇️

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[willow-ahrens]

I'm curious, on my machine I'm noticing that the llvm approach is faster than the atomic_pointermodify approach. Is this expected? Do you plan to continue using this package as Julia begins to provide more atomic operations?