Convert structs that implement the Enumerable protocol to lists

[andersonmcook]

I found it surprising that I could not use a MapSet or another Enum when using in in a query as I expected it to function similarly to Kernel.in/2.

ids = MapSet.new([1, 2, 3])
where(Comment, [c], c.id in ^ids) 

This PR checks for an implementation of the Enumerable protocol and converts it to a list when casting, returning an error if there is none.

If this isn't the direction you want Ecto to go, I'm happy to have this closed. Otherwise, let me know anything else I should be mindful of or any documentation I should update.

Thanks!

[greg-rychlewski]

Thanks. It would be good to update the docs here: https://hexdocs.pm/ecto/Ecto.Query.API.html#in/2

[josevalim]

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