Add a simple tokio-based nursery

[jwodder]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 92.30769% with 5 lines in your changes missing coverage. Please review.

Project coverage is 82.19%. Comparing base (ae88c3b) to head (fb3edd0).
Report is 43 commits behind head on master.

Files with missing lines	Patch %	Lines
crates/tokio-nursery/src/lib.rs	92.30%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #29      +/-   ##
==========================================
+ Coverage   81.68%   82.19%   +0.50%     
==========================================
  Files          31       32       +1     
  Lines        1294     1359      +65     
==========================================
+ Hits         1057     1117      +60     
- Misses        237      242       +5     

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