containers: intersections for Set, along with Semigroup newtype

[Georgi Lyubenov]

I would be opposed to
intersections [] = []
as it is true that *all* things have the property of being a member of
every element of []. In set theory without classes I've usually seen this
worked around by never using intersections on an empty set (ie using
NonEmpty)

=======
Georgi
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