Allow Enum.zip/1 and Stream.zip/1 to work on Streams

[jgmchan]

Before v1.6.0 the following code worked:

 iex(1)> Stream.chunk([0, 1, 2, 3], 2) |> Stream.zip() |> Enum.to_list()
[{0, 2}, {1, 3}]

However, we now get an error in versions >=1.6.0:

iex(1)> Stream.chunk([0, 1, 2, 3], 2) |> Stream.zip() |> Enum.to_list()

** (FunctionClauseError) no function clause matching in Stream.zip/1

    The following arguments were given to Stream.zip/1:

        # 1
        #Stream<[
          enum: [0, 1, 2, 3],
          funs: [#Function<3.91433161/1 in Stream.chunk_while/4>]
        ]>

    (elixir) lib/stream.ex:1110: Stream.zip/1

This happens with Enum.zip/1 as well.

It looks like the behaviour of the zip/1 function was changed in #6322.

If this is not the intended behaviour then this PR will allow the pre-v1.6.0 code to work again. Otherwise, I'm happy to close this PR and update the documentation to state that zip/1 can only work with lists of enumerables and not streams.

[jgmchan]

This would be much cleaner if we can have a guard that checks that enumerable is a list or a %Stream{} struct but I'm not sure how to do that.

[jgmchan]

This is a a pretty bad name but I can't think of a better one 😄 . I thought about calling it _zip but I think it seems to clash with the convention of unimported functions. Any advice on this would be great!

[jgmchan]

Hmm, failed tests. Will look into it.

[jgmchan]

Used pattern matching for the __struct__ key because clean compilation fails if I use this:

def zip(%Stream{} = enumerables), do: do_zip(enumerables)

I'm wondering if we need to check the enumerable type here if Stream.zip/1 already does it.