Aggregate functions cannot be nested, and this is validated during the
translation of aggregate function arguments. Therefore, traversing their
child expressions is unnecessary.
This bug occurred when arguments were read for the GROUP BY sorter — all
arguments were incorrectly resolved to the first column. Added tests
confirm that aggregates now work correctly both with and without the
sorter.
`BTreeTable::to_sql` makes us incompatible with SQLite by losing e.g. the original whitespace provided during the CREATE TABLE command.
For now let's fix our tests by regex-replacing every CREATE TABLE in
the entire repo to have exactly 1 space after the table name in the
CREATE TABLE statement.
