Unify `FilteredQuerySource` and `SelectStatement`

[sgrif]

The reason FilteredQuerySource exists is basically to say "select was never called explicitly". This is because we want update(users.filter(id.eq(1))) to work, but not update(users.select(id)), since that's nonsense.

Right now, SelectStatement first comes into play at the AsQuery level. A table will implement AsQuery by doing SelectStatement::simple(all_columns, self). Rather than passing all_columns there, I'd like to introduce some sort of "default select clause" marker, and then only implement UpdateTarget for SelectStatement when the select clause is that marker.