add IsList instance to Data.Sequence

[David Luposchainsky]

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I just looked at Set, and it doesn't seem to be an instance of IsList as
well (in 0.5.5.1). Could it be that `containers` simply hasn't been
taken into account yet? Set and Seq clearly support IsList.

So +1 and all that.

Greetings,
David/Quchen

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