Allocate a contiguous region of virtual memory to be subdivided among

Allocate a contiguous region of virtual memory to be subdivided among
the various heaps managed by the garbage collector.  Because we cannot
tell how far the break has been advanced by morecore, we over allocate
virtual memory and grain each heap on a multiple of the maximum heap
size.  If we could reckon the position of the break, we could allocate
just as many pages as required.  This requires exporting more state
from mspace.c, a refinement I will reserve for a future change list.