Table names seem to get dropped from joins etc.

[abloom]

While working on some queries for my team I noticed that table names weren't present in my join statements and occasionally this was causing issues with ambiguous column names.

I started to look through the code and think the issue is in some old code in the visit_column function. I've opened PR #97 which then uncovered what I suspect is another bug involving types. I can open a separate issue/PR for that if you prefer.

I understand PR #97 may not be the right solution, I admit the code is not entirely clear to me even after looking at the commit history of the relevant lines. I would be happy to submit a better solution but may need some help understanding the implementation.