Pausing garbage collection

[Jonathan A Rees <JAR@AI.AI.MIT.EDU>]

The main reason T uses Clark's GC is to linearize lists and thereby
reduce paging, presumably.  (Also because we had just spoken to Clark
before we wrote it.)  I'm not sure it's otherwise a win; I think it may
inspect objects objects fewer times, but the constant factor might be
bad because of the way it represents its stack -- several memory
references are required to chase forwarding pointers and get the next
object to inspect.  The usual scan/free pointer copying GC (I forget
what it's called) is certainly simpler and probably just as good,
especially if you have lots of physical memory.  It would be nice to see
some experiments comparing the two, however.