Work around greenlet 0.4.17's misguided contextvars support

[oremanj]

greenlet 0.4.17 added support for switching contextvars when switching greenlets. Great! Except... each greenlet now starts with an empty context, and trying to work around this with Context.run() tends to result in problems not keeping the implicit stack of Context.run() calls properly nested (since if you switch away from a greenlet, any Context.run() it was in the middle of still appears to be active). See https://github.com/python-greenlet/greenlet/issues/196 for more details.

Work around this by using ctypes to change the new greenlet's context pointer to be the same as the old one before switching. This probably comes with some performance cost, but I don't see a better alternative.

[codecov[bot]]

Codecov Report

Merging #5 into master will increase coverage by 0.40%. The diff coverage is 98.71%.

@@            Coverage Diff             @@
##           master       #5      +/-   ##
==========================================
+ Coverage   99.42%   99.82%   +0.40%     
==========================================
  Files           7        7              
  Lines         518      585      +67     
  Branches       37       42       +5     
==========================================
+ Hits          515      584      +69     
+ Misses          2        1       -1     
+ Partials        1        0       -1     

Flag	Coverage Δ
#cpython	99.14% <98.71%> (+0.49%)	:arrow_up:
#pypy36nightly	89.23% <45.45%> (-4.49%)	:arrow_down:
#pypy36release	89.23% <45.45%> (-4.49%)	:arrow_down:

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Impacted Files	Coverage Δ
greenback/_impl.py	99.40% <97.95%> (+1.71%)	:arrow_up:
greenback/_tests/test_impl.py	100.00% <100.00%> (ø)

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