Part 1 of union support. Adding it to the OperatorAssembler.
Refactor some the two Union operators into common UnionBase operator.
Fix the assembleUnion and ordering construction for the Union operators
Add more tests, update docs for same.
Update the tests.
Add test files
add one more missing sql file
Add a UnionAllIT test to verify Union operators work with 
two different tables, and still get correct results.
Add another test case (and fix it) to unions. Types in two sides have to
match.
Add tests for mismatches on row types (same number, not compatible
types).
Add recommended change to AISBinder to resolve multiple unions and
select *. Add tests to verify this works as expected.
Fix ASTStatementLoader to add a ResultsNode to allow the
Union to assemble correctly. Breaks tests not fixed here.
Add YAML test. This doesn't work currently because Union return bad
results.
Merge from trunk
Clean up the merge from trunk in the conflicted files.
Add more tests for the Union, then fix the code uncovered as broken.
Add error support. Expand tests for errors.

Doing a sort by null constant fails. So add test, verify and fix.
more test fixes.
Add Explain for UNION ALL
New round of tests. Add some implicit casting of types to avoid user
having to cast NULL to VARCHAR(32) (or similar explicit casting). There
are limits (VARCHAR vs INT errors, for example).