Set operators

[verdverm]

Set operators on structs could be useful.

Extending them to lists could prove difficult, particularly lists of structs (which are difficult for all operations). Lists of basics could be done with a package (Go generics helpful here?).

• intersection
• difference
• symmetric difference
• subset test
• superset test
• others?

(taken from cue-lang/cue#454)

[verdverm]

union & intersection seem to have overlap with insert/upsert & pick respectively

maybe super/subset could be called contains/containedby?