GC Sentinel

[dsimcha]

I'm looking at the GC implementation and starting my hacking.  I noticed that
the ends of blocks are already being used in some creative ways in the
sentinel version.  This looks like a debugging feature, though I don't
understand it completely.  Can one of the original implementors (Walter, Sean)
explain to me what the heck version(SENTINEL) in gcx.d does at a high level,
whether it conflicts with storing bit masks for precise heap scanning at the
ends of memory blocks, and whether we still even need it?