Dump all strings/symbols?

["Research" <research@...>]

I sorta asked this in reply to another question, but I still don't know how 
to do it.

Is there a way to traverse the entire globals/registry and get the 
strings/symbol names out? (only top level stuff is needed, not a deep 
traversal) We're setting tables and functions to nil and it seems that not 
everything is being garbage collected. I want some way to find out what lua 
is still holding references to so I can clean them up. What are the ways to 
do this?

Thanks,
Brett