Implement happy eyeballs

[njsmith]

Not tested, not integrated, just want to get it off my laptop.

[codecov-io]

Codecov Report

Merging #145 into master will increase coverage by 0.01%.
The diff coverage is 99.41%.

@@            Coverage Diff             @@
##           master     #145      +/-   ##
==========================================
+ Coverage   99.14%   99.15%   +0.01%     
==========================================
  Files          63       67       +4     
  Lines        8849     9189     +340     
  Branches      633      656      +23     
==========================================
+ Hits         8773     9111     +338     
- Misses         61       62       +1     
- Partials       15       16       +1

Impacted Files	Coverage Δ
trio/ssl.py	100% <ø> (ø)	⬆️
trio/tests/test_ssl_helpers.py	100% <100%> (ø)
trio/_ssl_stream_helpers.py	100% <100%> (ø)
trio/__init__.py	100% <100%> (ø)	⬆️
trio/tests/conftest.py	100% <100%> (ø)	⬆️
trio/tests/test_open_tcp_stream.py	100% <100%> (ø)
trio/_open_tcp_stream.py	96.66% <96.66%> (ø)
... and 1 more

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[njsmith]

This is now working, and has comprehensive tests!

The docs are a mess; I just shoved the new functions in somewhere that isn't too terrible, but really that whole section needs to be reorganized soon. Maybe better to wait until we have the server stream APIs working though? I dunno.