Use cons-cell based memory model
Fix off-by-one in DIE()
Add mmap and munmap syscalls
Store numbers bitwise negated
Save and restore values across potential GCs
Make quote allocate a new cell

This means that pointers now can't point to the middle of a cell. Also,
we're not keeping unreferenced subtrees alive.
Add GDB plugin for printing ABC values
Allocate heap with mmap, rewrite pointers when heap moves
Add basic 2-space garbage collection