Fixity declarations for member, notMember, union, intersection

As discussed at issue #47. I added the following declarations to Data.Set, Data.IntSet, Data.Map, and Data.IntMap:

infix 4 member
infix 4 notMember
infixl 5 union
infixl 5 intersection

I did not add an infix declaration for ``difference``, because there already exists the `\\` operator, and it is declared "infixl 9". I don't think "infixl 9" is very logical (I would have preferred `\\` to have the same precedence as `++`), but I also didn't want to change an existing declaration, nor declare ``difference`` to have a different fixity than `\\`. Therefore, I have left ``difference`` unchanged, with no fixity declaration, which means it defaults to "infixl 9" anyway.