Add join support and calling functions on functions

[spiegela]

I know it's been a while since this program has been updated, and I'm not sure if it's still in use. I've found it very useful for working with a legacy schema that doesn't fit into other tool requirements. Thanks for making it available!

I've added join support for a project I'm working on.

I've also added the ability to call functions on functions for some weird SQL like:

SELECT
  CONCAT('[',GROUP_CONCAT(comment.id),']')
FROM posts
JOIN  comments AS comment
  ON comments.post_id = posts.id
GROUP BY posts.id

[ptrf]

This PR fails with the following test case:

{<<"SELECT * FROM outer(var1 := 1, var2 := inner(innervar := 2))">>,
 ?_safe_test({select,'*',{from,{call,outer,[{var1, 1},{var2,{call,inner,[{innervar, 2}]}}]}}})
}

[spiegela]

@ptrf Can you post the stack trace of the failure to gist or somewhere?

[ptrf]

@spiegela Here you go: https://gist.github.com/ptrf/b6179a092f17a8d3c5a1

This is from running make test in the current master HEAD with the additional test case written out.

Do also read the 3 commits and commit messages attached in PR #6 that I submitted to fix this issue.