server: column name function (#730)

We'll need this later for sema stuff

If we have:

```sql
with t as (select 1)
select * from t;
```

We need to figure out the column name from the expr in `select 1` to support analysis of the statements later on.

Our parser does some weird stuff with some casts and there's also this fallback thing with inferred column names in postgres that also makes this more complicated than it should be.

Author: sbdchd