Add UNION/INTERSECT/EXCEPT support for compound SELECT

[renecannao]

Support compound queries with full recursive nesting:

• UNION [ALL], INTERSECT [ALL], EXCEPT [ALL]
• Parenthesized compound queries with precedence (INTERSECT binds tighter)
• Trailing ORDER BY / LIMIT on compound result

Depends on: SELECT parser (done)

[renecannao]

Implemented in PR #4 (Plan 10). UNION/INTERSECT/EXCEPT with precedence and parenthesized nesting.