Source location improvements for Allegro CL >= 10.0

Hello, Allegro CL 10.0 features improved source location information for compiler warnings: whereas previously warnings were associated with the top-level form where they occur, now we can pinpoint the offending subform. I've just pushed some code to the master branch that takes advantage of this enhanced information. (screenshot: http://imgur.com/pe6Bw20) Feedback from Allegro users is welcome! Cheers, Luís