Introduce a collection_constructor (similar to C++'s std::initializer_list)
to hold subexpressions being gathered into a list. Since sets, maps, and
lists construction share some attributes (all elements must be of the
same type) collection_constructor will be used for all of them, so it
also holds an enum. I used "style" for the enum since it's a weak
attribute - an empty set is also an empty map. I chose collection_constructor
rather than plain 'collection' to highlight that it's not the only way
to get a collection (selecting a collection column is another, as an
example) and to hint at what it does - construct a collection from
more primitive elements.
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