fix regression in expression printing (ref #5879)

fix #5898, showing 1-arg form of kw expr (used internally)

add prettyprinting of string interpolation
  without this, test-show fails on my fix to #5879, which helps explain
  why things were that way
Merge branch 'master' of github.com:JuliaLang/julia into jb/setconstructor
NEWS and doc updates for collection constructor changes