Allow Generators for vol.In

[cdce8p]

While the use of Container for vol.In is correct, it's incompatible with Generator even though the ... in ... check works fine.

For objects that don’t define contains(), the membership test first tries iteration via iter()

Ref #518
https://docs.python.org/3/library/collections.abc.html#collections-abstract-base-classes
https://docs.python.org/3/reference/datamodel.html#object.__contains__

/CC @farmio

[farmio]

Nice! Thanks.

[alecthomas]

Awesome, thanks.