Compound statements with variable number of select statements

[trueqbit]

This PR extends the operators for compound statements (i.e. union_, union_all, intersect, except)

• to accept a variable number of single select statements, which is especially useful for UNION ALL,
• to allow the select statements to return a common type.

Additionally:

• Simplified code for the calculation of node tuples.
• Improved unit tests for node tuples.

My motivating use case is to be able to handpick multiple value ranges:

struct Object {
    int64 id;
    int64 difference;   // diff value in days
};

union_all(
    select(cast<int64>(min(&Object::difference))),
    select(count(), where(c(&Object::difference) < 0)),
    select(count(), where(c(&Object::difference) >= 0 and c(&Object::difference) < 7)),
    select(count(), where(c(&Object::difference) >= 7 and c(&Object::difference) < 15)),
    ...
);