detect ambiguous columns

[cyberhuman]

This is accepted by sqlgg:

create table foo ( id integer primary key );
create table bar ( id integer primary key );
select * from foo join bar on foo.id = bar.id order by id;

but fails in run time (mysql) with:

ERROR 1052 (23000): Column 'id' in order clause is ambiguous

[jongleb]

https://github.com/ygrek/sqlgg/pull/21