Use as rather than map and discard

[johnynek]

I noticed a few places where we do map(f)(_ => a). This is the same as Functor.as, but it hides it from the typeclass implementation. Some types can optimize as by discarding any trailing map operators from their internal AST (such as parsers or other compute types that are not evaluated until later). By not calling typeclass methods we potentially slow those implementations down.

[codecov-io]

Codecov Report

Merging #3708 (26e16e5) into master (8e8be51) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3708   +/-   ##
=======================================
  Coverage   90.24%   90.24%           
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  Files         391      391           
  Lines        8863     8863           
  Branches      270      227   -43     
=======================================
  Hits         7998     7998           
  Misses        865      865           

[rossabaker]

A scalafix linter for patterns like this, or .pure(()), would be neat.