Add Enum.concat/1,2 and Stream.concat/1,2
Deprecate List.concat/1,2
Replace occurrences of List.concat/1,2 with Enum.concat/1,2
Move the List concat tests to Enum
Add support for enumerables of enumerables
Add tests for Stream.concat/1,2 and Enum.concat/1,2
Merge pull request #1660 from betawaffle/enum-concat

Add Stream.concat/1,2 and Enum.concat/1,2
Add note to Enum to clarify what's returned

In particular clarify that infinite streams don't work with many of the
functions.
Merge pull request #1662 from pminten/note-about-enum-list-return

Add note to Enum to clarify what's returned
Improve Enum docs
out of it -> as result
Make a distinction about eager and full evaluation of the collection