turn on deprecation warnings for type and immutable

[JeffBezanson]

No description provided.

[tkelman]

should probably backport the doc and test bits of this, obviously not the deprecation itself though

[JeffBezanson]

Commit split for that purpose.

[tkelman]

there was one more under examples that I fixed for you, squashed into the first commit

(backport pending only for that commit)

[KristofferC]

FWIW I updated https://gist.github.com/KristofferC/24c269727d914ffb88bfc21624dad891.

Run update_folder!(folder::String) to see what files that would be edited and then update_folder!(folder, false) to actually do the replacements. I did
KristofferC/NearestNeighbors.jl#46 and KristofferC/Crayons.jl#10 with the script.

[ararslan]

I probably would have waited until there's a 0.6 release so that packages can soundly require 0.6 without having to worry about 0.6-, which can include 0.6-dev pre-struct. ¯\_(ツ)_/¯