This replaces the previous virtual address allocator which was basically
just "m_next_address += size;"
With this in place, virtual addresses can get reused, which cuts down on
the number of page tables created. When we implement ASLR some day, we'll
probably have to do page table deallocation, but for now page tables are
only deallocated once the process dies.
This means that kernel regions will eagerly get physical pages allocated.
It would be nice to zero-fill these on demand instead, but that would
require a bunch of MemoryManager changes.
This patch moves away from using kmalloc memory for thread kernel stacks.
This reduces pressure on kmalloc (16 KB per thread adds up fast) and
prevents kernel stack overflow from scribbling all over random unrelated
kernel memory.
