Unify NULLIF scalar, IFNULL and IF argument type resolution to picking.

IF, NULLIF, IFNULL and COALESCE all be have the same (with respect to argument types) now.

Depends on FoundationDB/sql-parser#159
Casted to a double, yes.

```
test=> SELECT CAST(DATE '1991-12-12' AS DOUBLE);
 _SQL_COL_1
-------------
 1.9911212E7
(1 row)
```