parser: Support columns from subselect statements - #489

Cool that it works, however there is still an issue when using joins in a subselect statement.
Take this example:

SELECT r.*
FROM (
        project_saves
        JOIN projects
        ON project_saves.project_id = projects.id
    ) r;

This fails with the message

# package sqlc
sql-queries/projects.sql:159:1: empty edit contents

sqlc seems to believe that thesubselect r has no columns since when selecting a field of r the error message is "column "id" does not exist". Is there an easy way to fix this?

Also, subquery aliasing does not work when sqlc.arg is used

SELECT *
FROM (SELECT * FROM report LIMIT ...) AS r
JOIN ... 
WHERE r.created_at >= sqlc.arg('since')

table alias "r" does not exist

(I need to do pre-filtering, if I do a flat join, MySQL runs out of sort memory)