Chicken Cairo update

I've updated chicken-cairo to keep up with a number of changes in the Cairo API. I also shifted it to darcs — GNU/Arch is just too much of a martinet for me.

$ darcs get http://eighty-twenty.org/~mikeb/Darcs/chicken-cairo

I came across an odd problem using Chicken Scheme while I was trying out my updates: I used records to wrap foreign pointers, so I could check types and provide record printers; however, it seems that these pointers are moved around (during garbage collection), meaning that they are trashed in-between getting them from a C function invocation and using them. Using tagged pointers instead of records appears to avoid this problem. I don't understand very well why this is so, yet.

The code contains annotations, where useful and I've remembered, as to what's changed in the Cairo API.