Generational/incremental garbage collector

   o  The generational/incremental garbage collector still is
      considered experimental, although it stands up well in some real
      applications.

   o  On a Sun (Sun-4/SunOS4 or 5) when compiling Elk with gcc (2.5.4
      or 2.5.7), the generational garbage collector sometimes loops
      (when working in non-incremental mode).  This can be
      circumvented by compiling src/proc.c without the -O option.  We
      are not sure yet whether this is a bug in Elk or in gcc.

   o  Running out of memory when expanding the heap shouldn't be handled
      as a fatal error.  Instead, the garbage collector should clean up
      and then invoke UncatchableError() to return control to the Scheme
      program.

   o  The return value of ExpandHeap() is sometimes ignored.

   o  When running the program

	 (garbage-collect-status 'generational 'incremental)
	 (define (f) (make-list 10000 'a) (f))
	 (f)

      the pairs in the lists become stable quickly and aren't
      reclaimed, as the current algorithm favors heap expansions over
      full collections.

   o  With the same test program, the GC sometimes crashes with SIGSEGV
      after having expanded the heap to 9MB.

   o  Under HP-UX 9.0, the GC doesn't work in incremental mode (a
      broken-heart is passed to Memoize_Frame() after an ExpandHeap()).

   o  The percentage displayed at the end of a GC run is sometimes
      wrong.
