Define upreferred(::TwicePrecision)

[sostock]

Closes #564.

[codecov-commenter]

Codecov Report

Merging #577 (fbcbe5c) into master (ebad449) will increase coverage by 0.15%.
The diff coverage is 100.00%.

❗ Current head fbcbe5c differs from pull request most recent head b7475de. Consider uploading reports for the commit b7475de to get more accurate results

@@            Coverage Diff             @@
##           master     #577      +/-   ##
==========================================
+ Coverage   87.92%   88.07%   +0.15%     
==========================================
  Files          16       16              
  Lines        1449     1451       +2     
==========================================
+ Hits         1274     1278       +4     
+ Misses        175      173       -2     

Impacted Files	Coverage Δ
src/range.jl	92.94% <100.00%> (+1.37%)	⬆️
src/user.jl	95.78% <0.00%> (+0.52%)	⬆️

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