Remove Core.Box

[jwahlstrand]

Use JuliaLang/julia#60478 to find cases where Core.Box is produced by the compiler. Fix by introducing local variables.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 84.37500% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.10%. Comparing base (56cd523) to head (b70516e).
⚠️ Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
src/widgets.jl	83.33%	3 Missing ⚠️
src/extrawidgets.jl	85.71%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #79      +/-   ##
==========================================
- Coverage   92.49%   91.10%   -1.39%     
==========================================
  Files           6        6              
  Lines        1119     1158      +39     
==========================================
+ Hits         1035     1055      +20     
- Misses         84      103      +19     

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