xo-alloc -- arena allocator and incremental garbage collector

Rules for writing garbage-collected classes.

Topics

allocation - allocate Objects (inheriting xo::Object) before owned scratch space. Can relax this if/when abandon the bad-for-locality use of two pointers into to-space to keep track of grey objects. Want to use stack anyway so we can do depth-first search.
destructors - can omit except for finalization
assignment - MUST USE Object::assign_member() to assign pointers to gc-owned memory. Only necessary for old->new pointers, so don't need to worry about this for initialization.
finalization - not supported (yet)