Remove everything else that was deprecated in 0.2.0

[njsmith]

Closes #68, #321, #317, #157, #291.

[codecov]

Codecov Report

Merging #389 into master will increase coverage by 3.39%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #389      +/-   ##
=========================================
+ Coverage   95.91%   99.3%   +3.39%     
=========================================
  Files          87      87              
  Lines       10334   10259      -75     
  Branches      714     712       -2     
=========================================
+ Hits         9912   10188     +276     
+ Misses        376      56     -320     
+ Partials       46      15      -31

Impacted Files	Coverage Δ
trio/_threads.py	100% <ø> (ø)	⬆️
trio/tests/test_threads.py	100% <ø> (ø)	⬆️
trio/tests/test_socket.py	100% <ø> (+10.18%)	⬆️
trio/_sync.py	100% <ø> (ø)	⬆️
trio/__init__.py	100% <ø> (ø)	⬆️
trio/_toplevel_core_reexports.py	100% <ø> (ø)	⬆️
trio/_socket.py	100% <ø> (+9.76%)	⬆️
trio/testing/__init__.py	100% <ø> (ø)	⬆️
trio/tests/test_sync.py	100% <100%> (ø)	⬆️
trio/_core/_multierror.py	99.38% <100%> (+3.57%)	⬆️
... and 30 more

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