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