UNION form

`UNION` is currently not a statement but a `table-expr` - this means that the following expression fails.

`(sql (union (select x #:from xs) ...))`.

This isn't too much of an issue since you can wrap it in a second `select`; but when dealing with something like [`with` recursive](https://sqlite.org/lang_with.html) it becomes harder.

Am I missing something?

