ENH: Add Cary Flag check to RDRAND

[bashtage]

Add check of carry flag and raise RuntimeError if not successful
Add retry variable

closes #246

[codecov]

Codecov Report

Merging #247 into master will decrease coverage by 0.01%.
The diff coverage is 92.71%.

@@            Coverage Diff             @@
##           master     #247      +/-   ##
==========================================
- Coverage   95.20%   95.18%   -0.02%     
==========================================
  Files          57       57              
  Lines       13051    13187     +136     
  Branches      235      235              
==========================================
+ Hits        12425    12552     +127     
- Misses        605      614       +9     
  Partials       21       21              

Impacted Files	Coverage Δ
randomgen/_testing.py	100.00% <ø> (ø)
randomgen/common.pyx	80.94% <ø> (ø)
randomgen/entropy.pyx	84.61% <ø> (ø)
randomgen/seed_sequence.py	100.00% <ø> (ø)
randomgen/tests/test_stability.py	97.95% <0.00%> (ø)
randomgen/rdrand.pyx	87.80% <89.24%> (+4.02%)	⬆️
randomgen/tests/test_direct.py	99.72% <100.00%> (+<0.01%)	⬆️
randomgen/tests/test_generator_117.py	100.00% <100.00%> (ø)
randomgen/tests/test_lcg128mix_pcg64dxsm.py	96.83% <100.00%> (ø)
randomgen/tests/test_randomstate_regression.py	99.06% <100.00%> (ø)
... and 2 more

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