expand subquery column lists first before macro expansion

[riastradh-probcomp]

Currently we expand subquery column lists -- SELECT t.(SELECT ...) -- at the same time we format the SQL output. The ad hoc macro expansion for SIMULATE <compound expression> FROM MODELS OF p that I added last week happens before that, but for SIMULATE t.(<subquery>) FROM MODELS OF p to work, we need the macro expander to know the list of variables produced by first. So the easiest way to accomplish that will be to do things in three stages:

1. Expand subquery column lists.
2. Expand compound expression SIMULATE into SELECT of primitive expression SIMULATE.
3. Generate SQL.

[riastradh-probcomp]

Done in f764732.