We're targetting systems that need to be able to inspect the kind of an
object, so this information has to be somewhere. If it's out-of-line,
we might save memory, but we would lose locality. Concretely in Guile
the tag bits are in the object itself.
Also expand GC interface with "allocate_pointerless". Limit lazy
sweeping to the allocation size that is causing the sweep, without
adding to fragmentation.
