Implement IDNA encode/decode caching

[asvetlov]

No description provided.

[codecov]

Codecov Report

Merging #476 into master will increase coverage by 0.01%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master     #476      +/-   ##
==========================================
+ Coverage   99.25%   99.27%   +0.01%     
==========================================
  Files           2        2              
  Lines         673      689      +16     
  Branches      154      154              
==========================================
+ Hits          668      684      +16     
  Misses          5        5              

Impacted Files	Coverage Δ
yarl/__init__.py	99.04% <91.66%> (+0.03%)	⬆️

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