Add task- and run-local storage.

[njsmith]

Fixes #2

[codecov]

Codecov Report

Merging #179 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #179      +/-   ##
==========================================
+ Coverage    98.4%   98.44%   +0.03%     
==========================================
  Files          51       53       +2     
  Lines        5963     6116     +153     
  Branches      470      476       +6     
==========================================
+ Hits         5868     6021     +153     
  Misses         80       80              
  Partials       15       15

Impacted Files	Coverage Δ
trio/_core/_local.py	100% <100%> (ø)
trio/_core/__init__.py	100% <100%> (ø)	⬆️
trio/_core/_run.py	100% <100%> (ø)	⬆️
trio/_core/tests/test_local.py	100% <100%> (ø)

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