This is done by creating a cursor for each set. This is a cursor for
the actual values in the sets. We then pick the "smallest" value from
the cursors and advance that cursor. This continues until we have
exhausted all the cursors.
setA.Union(set0, ... setN)
The time complexity is O(len(setA) + len(set0)) + ... len(setN))
The generated code for typed structs now uses a Go struct which
implements Value directly. The fields in this struct uses the "user"
type. (The union value still uses types.Value though.)
When a typed struct is created by the decoder, it asks for a struct
builder which returns a channel that the values of the fields of the
struct are sent to.
