Change the GC so that it can mark and sweep objects in-place, instead
of always copying. This change is helpful for reducing peak memory
use while performing a collection on a large, old heap.
Some non-copying support was already in place for locked objects,
but the new implementation is faster and more general. As an
alternative to locking, the storage manager now provides "immobile"
allocation (currently only for bytevectors, vectors, and boxes),
which allocates an object that won't move but that can be GCed if
it's not referenced. A locked object is an object that has been
immobiled and that is on a global list --- mostly the old,
non-scalable implementation of locked objects brought back, since
immobile objects cover the cases that need to scale.
original commit: aecb7b736cb1d52764c292fa6364a674958dfde3
