Changed Checked.Shell implementation to rely on extend and not type aliasing

[soronpo]

Gets around Scalac issue scala/bug#10372

[codecov]

Codecov Report

Merging #38 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #38      +/-   ##
==========================================
+ Coverage   97.52%   97.54%   +0.01%     
==========================================
  Files          11       11              
  Lines        1174     1180       +6     
  Branches        6        6              
==========================================
+ Hits         1145     1151       +6     
  Misses         29       29

Impacted Files	Coverage Δ
...n/scala/singleton/twoface/impl/Checked1Param.scala	100% <100%> (ø)	⬆️
...n/scala/singleton/twoface/impl/Checked0Param.scala	100% <100%> (ø)	⬆️
...in/scala/singleton/twoface/impl/CheckedShell.scala	100% <100%> (ø)	⬆️

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