Refactor src/Base.hs.
Refactor src/Base.hs.

* Get rid of polyvariadic function for better readability and robustnes.

* Eliminate joinArgs and joinArgsSpaced functions. Users are
  encouraged to use 'unwords <$>' and 'concat <$>' instead.

* Generalise filterOut function.

* Rename ShowAction to ShowArgs.

sdiehl/ghc,snowleopard/hadrian,sdiehl/ghc,snowleopard/shaking-up-ghc,sdiehl/ghc,quchen/shaking-up-ghc,sdiehl/ghc,izgzhen/hadrian,bgamari/shaking-up-ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc