Code GC

I've started work on a garbage collector for the compiled code heap. A code block is eligible for reclamation when the word owning the code block is no longer referenced in the data heap, and when no other compiled code blocks reference the code block. So if you redefine a compiled word, or forget a word, the old compiled block will be reclaimed.

Code GC will use the mark and sweep algorithm, without moving the code blocks around, in contrast to the generational copying GC used for data. The reason code cannot be moved around reliably is callbacks -- C code may hold on to pointers to Factor code. The heap will be compacted when the image is saved.

Of course if a callback refers to a word which has been GC'd, your program will crash if it attempts to invoke the callback. The solution is "don't do it, then" -- if a callback is actively held on to by C code, the consequences of forgeting or redefining it are undefined.