ENH: Add CPU dispatch for ChaCha

[bashtage]

Add CPU dispatch for SIMD

[codecov]

Codecov Report

Merging #175 into master will decrease coverage by 0.06%.
The diff coverage is 61.11%.

@@            Coverage Diff             @@
##           master     #175      +/-   ##
==========================================
- Coverage   95.08%   95.02%   -0.07%     
==========================================
  Files          37       37              
  Lines       10118    10134      +16     
  Branches      151      152       +1     
==========================================
+ Hits         9621     9630       +9     
- Misses        474      480       +6     
- Partials       23       24       +1

Impacted Files	Coverage Δ
randomgen/mtrand.pyx	91.14% <100%> (ø)	⬆️
randomgen/generator.pyx	92.77% <100%> (ø)	⬆️
randomgen/tests/test_direct.py	98.68% <55.55%> (-0.35%)	⬇️
randomgen/chacha.pyx	88.99% <57.14%> (-2.19%)	⬇️

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