Use :stdio instead of group_leader() as default device

[michalmuskala]

As discussed on IRC, I'm opening this PR as a place for further discussion on the issue.

This commit basically reverts the change introduced in: 249f83e

Using :stdio gives us much better error in case the data is not
printable, and the function signature is more understandable when
accessed with h/1.

As an example of better error message let's look at the error before the change:

iex(1)> IO.puts([:io])
** (ArgumentError) argument error
    (stdlib) :io.put_chars(#PID<0.50.0>, :unicode, [[:io], 10])

And after:

iex(1)> IO.puts([:io])
** (ArgumentError) argument error
    (stdlib) :io.put_chars(:standard_io, :unicode, [[:io], 10])

Notice the device is a PID when using group_leader() and a more understandable atom when using :stdio.

\cc @fishcakez

[josevalim]

I am a strong no on this feature. This means that, if I use remote shell, IO.puts won't be sent to the proper node. Won't it?

[josevalim]

I was wrong. :stdio is rewritten to group leader.

[josevalim]

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