Don't shuffle each batch, instead reverse them half the time.

[palkeo]

I consistently see batch shuffling taking 1% CPU on a real-world
application, and this is a simple fix to make this overhead disappear.
Suggested by @njsmith.

[codecov]

Codecov Report

Merging #1580 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1580   +/-   ##
=======================================
  Coverage   99.69%   99.69%           
=======================================
  Files         111      111           
  Lines       13866    13869    +3     
  Branches     1059     1060    +1     
=======================================
+ Hits        13824    13827    +3     
  Misses         27       27           
  Partials       15       15           

Impacted Files	Coverage Δ
trio/_core/_run.py	99.76% <100.00%> (+<0.01%)	⬆️
trio/tests/test_scheduler_determinism.py	100.00% <100.00%> (ø)
trio/tests/test_deprecate.py	100.00% <0.00%> (ø)

[pquentin]

Looks good to me, thanks!

I don't think we need a newsfragment here, since this isn't user-visible?

[trio-bot]

Hey @palkeo, it looks like that was the first time we merged one of your PRs! Thanks so much! 🎉 🎂

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